



































































































































































































































































































































































































































































































































































TM 




0 * 


Class 




Book 


Copyright N° 


COPYRIGHT DEPOSIT 















































































































NOTES ON ASSAYING 


AND 


METALLURGICAL LABORATORY 

EXPERIMENTS. 


BY 

RICHARD W. LODGE, 

Assistant Professor of Mining and Metallurgy , 
Massachusetts Institute of Technology. 



SECOND EDITION , REVISED. 


FIRST THOUSAND. 


0 

> 

y o 

> » 

) D > 


> > 5 


NEW YORK: 

JOHN WILEY & SONS. 

London: CHAPMAN & HALL, Limited. 

1906 



T H 5 S'O 



LIBRARY of CONGRESS 
Two Copies Received 

FEB 5 1906 

(j Copyright Entry 
'CLASS CZ XXc. No. 

13 7 18 > 

COPY B, 


Copyright, 1904, 1906, 

BY 

R. W. LODGE. 


/ 


/O 


f 

C t 


( 


ROBERT DRUMMOND, PRINTER, NBW YORK# 




PREFACE. 


In this book I have combined the notes which have been in 
use for many years by the third-year students in Assaying and 
part of the notes used by the fourth-year students in the Metal¬ 
lurgical Laboratory of the Massachusetts Institute of Technology. 

Many new data and experiments carried out by my assistants, 
by former students, and by myself, which have been accumulating, 
have now been added. The notes are written especially for the 
use of the students of the Institute and for those who are com¬ 
mencing assaying, but it is hoped that persons well versed in 
laboratory work and actual practice may also derive some infor¬ 
mation from them. At the laboratory, in the Institute, students 
are expected to use these notes on the work assigned, preceding 
and accompanying which lectures are given. 

In treating of the assay for the metals, I have endeavored to 
give first what I consider the best method or methods, with re¬ 
agents, the amount used, and the reasons for using them. The 
student thus becomes familiar with my way of working. After 
this are given the methods used or recommended by others. 

The reagents and fluxes are given as Mitchell groups them, 
for his method seems the simplest and most systematic. 

I have consulted the best works upon the subject, such as those 
by Berthier, Mitchell, Furman, Brown, Beringer, and Ricketts; 
any indebtedness to whom I wish to acknowledge. 

The manner of conducting the larger laboratory tests, here 
discussed, is based on many experiments and at present seems 
to be the best way to introduce students to metallurgical work 
on a large scale. 

I wish especially to thank for their loyalty and aid those 
who have been my assistants and many former students. 

R. W. Lodge. 


in 


Massachusetts Institute of Technology, 
September. L904. 









CONTENTS. 


CHAPTER I. 

Apparatus, Reagents, Materials. 

Balances. 

Weights. Assay Ton System. 

Memoranda as to Weights and Values. 

Assay Reagents. Reducing Agents. Oxidizing Agents. De¬ 
sulphurizing Agents. Sulphurizing Agents. Fluxes. 

Fusion Products. Slag. Matte or Regulus. Speiss. 

Furnaces and Fuels. 

Refractories. Fire-clays. Fire-brick. Crucibles. Graphite 
Crucibles. Scorifiers. Cupels. Muffles. 

Mortars and Lutes. 


CHAPTER II. 


Sampling. 

Labelling Samples. Methods of Sampling. Ores Carrying 
Metallic Particles. 

Concentration by Panning or Vanning. 


CHAPTER III. 

Assay of Ores for Silver. 

Scorification Method. Essentials in Process. Reactions. 

Amount of Lead Required for Different Ores. Rescorifying 
Buttons. Spitting of Ores during Scorification. Assay of Zinc 
Residues. 

Assay of Copper Matte. Assay of Copper and Copper Bars. 
Combination Wet and Dry Method. 

Cupellation. Reactions. Experiment with C.P. Silver. Sil¬ 
ver Losses. Effect of Different Temperatures. Effect of Varying 
the L*ad. Effect of Copper. Effect of Tellurium. 






VI 


CONTENTS. 


PAGE 

Crucible Method. 

Effect of Fluxes at High Temperature on Different Substances. 
Sulphates. Action in Presence of Litharge and of Lead Sulphide. 
Testing Rea 'ents. 

Fluxes and Reagents. 

Testing Reagents. Assay of Granulated Lead. Assay of Lith¬ 
arge. Oxidizing Power of Nitre. 

Reducing Agents. Fusion for Reducing Power. Influence of 
Other Reagents on this Fusion. 

Regular Assay. Silicious and Oxide Ores (Class I). Fusion 
in Pot-furnace. Fusion in Muffle-furnace. 

Sulphide Ores or Those with a Reducing Power (Class II). 
Preliminary Fusion for the Reducing Power. Influence of Reagents 
(Silica, Borax, Borax-glass, and Soda) on the Reducing Power. 
Effect of Temperature. 

Regular Fusion. Effect of Silica on the Fusion. Iron Method. 
Effect of Temperature. Ores containing Organic Matter. Size 
of Lead Buttons. Dusting of Ores. 

Special Methods. Copper Ores. Antimonial Ores. Pyrrhotite. 

Ores Carrying Barite. 


CHAPTER IV. 

Assay of Ores for Gold. 127 

Occurrence of Ores. Steps in Assay. 

Class I. Ores with no Reducing Power. Scorification Method. 
Crucible Method. Fusion in the Pot-furnace. Fusion in the 
Muffle. 

Class II. Ores with a Reducing Power. Scorification Method. 
Method of Roasting the Ore. Iron Method. Assay of Arsenical 
Ores. Litharge and Nitre Method. 

Class III. Telluride Ores. Effect of Tellurium upon Cupella- 
tion of Gold. 

Cupelling and Weighing the Beads of Precious Metals. 

Parting. Separation of Gold from Platinum and Iridium. 
Experiment in Roasting an Ore or Concentrates. 

Cupellation of Gold at Different Temperatures. Effect of 
Copper. 

Special Methods. Assay of Zinc-box Residues by Scorification. 
Crucible Assays. Copper Matte, Copper Bars, and Copper. Com¬ 
bination Wet and Dry Method. 

Gold and Silver in Antimony. Gold and Silver in Metallic 
Bismuth. 

Assaying Solutions. 

J O 





1 


CONTENTS. vii 

CHAPTER V. 

PAGB 

Assay of Ores for Lead. 190 

Lead Ores. 

Sulphide Ores. Fusion in the Muffle. Cyanide of Potash 
Method. Fusion in the Pot-furnace. Slags and Furnace Products. 

Ores Very Poor in Lead. Fusion in Iron Crucible. 

Ores Containing no Sulphides. 

General Remarks upon the Lead Assay. 


CHAPTER VI. 


Bullion. 198 

Lead Bullion or Base Bullion. 

Silver Bullion Containing no Gold. Silver Bullion Containing 
Gold. Wet Methods. 

Recovery of Silver from Solutions. 

Gold Bullion. 


CHAPTER VII. 

Assay of Ores for Copper and Tin. 213 

Copper Assay. 

Ores of Copper. Sulphide Ores. Oxide and Carbonate Ores. 
Native Copper Ores. 

Assay of Sulphide Ores (Class I). Roasting. Fusion. Re¬ 
fining Flux. 

Assay of Oxide and Carbonate Ores (Class II). 

Assay of Ores Carrying Native Copper (Class III). 

Assay of Ores for Tin. 

Tin Deposits. Ores. 

Steps in the Assay. Concentration. Roasting. Treatment with 
Acid. Panning. Assaying. 

Methods of Assaying. Cyanide of Potash Method. German 
Assay. Assay with Sodium Carbonate and Lime. Assay in Graphite 
Crucible. 


CHAPTER VIII. 

Platinum and the Platinum Group. 224 

The Platinum Group. 

Occurrence of Platinum. Sources. Qualitative Tests. Quan¬ 
titative Analysis. 






viii 


CONTENTS. 


. PAGE 

Table of Solubility of the Group. 

Assay of Sands and Ores. Tree of the Treatment. Determina¬ 
tion of Silver, Gold, Platinum, Iridium, and Iridosmium 

Platinum and Silver Alloys. Different Ratios of Platinum and 
Silver and their Solubility in Nitric Acid of Different Strengths. 

Platinum, Silver, and Gold Alloys and their Solubility in Nitric 
Acid. 

Platinum. Iridium. Palladium. Osmium. Ruthenium. 
Iridosmium. 


METALLURGICAL LABORATORY EXPERIMENTS AND 

NOTES. 243 

General Directions. 

Solutions. Calcining. Roasting. 

Chlorination of Gold Ores. Plattner Process. Effect of Impurities 
upon the Precipitation of Gold from AuC 1 3 . Barrel Process. 

Cyanide Process for Treatment of Gold Ores. Cyanide Process as 
Applied to Concentrates from Nova Scotia. Reactions in the Process. 
Testing a Roasted Ore for Sulphates. Alkali Wash. Poisoning. Potassium 
Cyanide. Titration of the Potassium Cyanide Solution. 

Treatment of Roasted Gold Ores by Means of Bromine. Cyanogen 
Bromide. 

Experimental Treatment of Gold-bearing Ores. Free-milling Test in 
Ball Mill. 

Amalgamation of Gold Ores. Stamp-mill Work. Making Silver 
Amalgam. Amalgams. Recovery of Silver and Mercury from the Nitrate 
Solution. 

Bullion. Melting and Refining. Toughening. Pouring and Casting. 
Small Amounts of Bullion. 

Retorting and Cleaning Mercury. 

Muffle Chloridizing Roast of Silver Ores. 

Pan Amalgamation of Ores. 




NOTES ON ASSAYING. 


CHAPTER I. 

INTRODUCTION. 

APPARATUS, REAGENTS, AND MATERIALS. 

Assaying is a branch of analytical chemistry generally defined 
as the quantitative estimation of the metals in ores, furnace 
products, bullion, coin, etc. This definition, however, makes no 
distinction between the results obtained by wet analysis and 
those obtained by fire. For instance, oftentimes we see the 
expression “assay of copper ores” or “wet assay for zinc,” mean¬ 
ing the determination of copper and zinc by some well-known 
wet method and not by fire. 

Assaying, strictly speaking, is the quantitative determination 
of metals in ores, furnace products, bullion, etc., by means of 
fire and dry reagents, and will be treated in this way in the follow¬ 
ing notes, except in some few cases where a wet method or a 
combination of a wet and a dry method is used. 

Assaying is chiefly applicable to mining and metallurgical 
operations where we wish to obtain accurate results in the shortest 
possible time. An assayer has generally to make a very large 
number of assays per day; whereas an equal number of chemical 
determinations would be out of the question. 

The student should realize at the beginning that neatness, 
care, and thorough attention to the work in hand are not only 



2 


NOTES ON ASSAYING. 


essential, but are perhaps more important than in chemical 
work. Careful observation is especially necessary. He should 
also realize that the amount of fluxes and reagents, which make 
up the various charges, is not a matter of guesswork, but each is 

i y 

used with a definite purpose in view. 

Balances.—In the student’s laboratory work three grades of 
balances seem absolutely essential; but one of these may perhaps 
be dispensed with in fitting up a laboratory for himself or for 
some mining company. 

i st. Flux-balance, capable of weighing 4 kilogrammes and 
sensitive to of a gramme; for weighing ore samples, fluxes, 
reagents, etc. 

2d. Pulp-balance , balance for weighing out the ore to be as¬ 
sayed, lead buttons from the lead assay, etc. It should be sen¬ 
sitive to ^ of a gramme, or 2 milligrammes. 

3d. Button-balance, for weighing the silver beads and the gold. 

This should be sensitive to A G f a milligramme. Balances of 
this character are the most sensitive and delicate in the world 
and the student should exercise the utmost care in the use of 
them. 

Weights.—For the above balances we have four sets of weights; 
but one of these may perhaps be dispensed with in the ordinary 
laboratory. 

1 st. (Flux-balance). One kilogramme to one gramme, for 
weighing ore samples, reagents, fluxes, etc. Additional kilo¬ 
gramme weights may be purchased to weigh larger samples. 

2d. (Pulp-balance). Twenty grammes to one centigramme, 
for weighing the ore for the lead, copper, and tin assays and the 
resulting button. 

3d. Assay ton weights, 4 A.T. to A.T., for weighing on 
the pulp-balance the ores to be assayed for silver and gold; also 
base bullion. 

4th. Set of fine weights, one gramme to one milligramme, to 
be used with the Button-balance. 

The 2d, 3d, and 4th sets of weights must never be handled 
with anything except the proper pliers. 

For these weights there are two places, and only two—in the 
scale-pan or in the weight-box. If they are placed anywhere else 





INTRODUCTION. 


3 


they are liable to get dust and other things on them. The 
student, owing to several using one balance, must realize that not 
only his own work, but that of others, depends upon the accuracy 
of the weights, and he should take every precaution in the use 
and care of them accordingly. 

The weights formerly used in assaying were grains, grammes, 
or fractions of these. 

Having our silver or gold accurately weighed, it was neces¬ 
sary to calculate from this weight and the weight of the sub¬ 
stance taken the percentage of silver or gold, and from this 
the number of ounces per ton. One per cent is equal to 291.66 
oz. troy in 1 ton avoirdupois of 2000 lbs. 

To avoid this amount of calculation, the assay ton (A.T.) 
system of weights was devised by Prof. C. F. Chandler of Colum¬ 
bia College, N. Y. 

All our ores and base metals are weighed in pounds avoirdu¬ 
pois; while the precious metals, gold and silver, are weighed in 
ounces troy. 

The basis of the A.T. system is the number of troy ounces 
(29,166+) in one ton of 2000 lbs. avoirdupois. 

1 ton avoirdupois = 2000 lbs. 

1 lb. “ =7000 grains troy (1 dram av. =27 n / 32 grains). 

Therefore 

1 ton avoirdupois = 14,000,000 grains troy. 

1 oz. troy = 480 grains. 

Therefore 


14,000,000 

480 


= 29,166+ oz. troy. 


One A.T. = 29,166 milligrammes or 29.166 grammes. That is, 
1 milligramme bears the same relation to 1 A.T. as 1 oz. troy 
bears to 1 ton of 2000 lbs. avoirdupois, or 

1 ton av. : 1 A.T. :: 1 oz. : 1 milligramme. 

Therefore, as soon as the student weighs his silver or gold, 
he can read the number of ounces troy that his ore runs per 
2000 lbs. avoirdupois. 



4 


NOTES ON ASSAYING. 


Ore Used. 

Button Obtained. 

Oz. Troy per Ton Av. 

i A.T. 

.00100 grammes 

I 

iA.T. 

•01536 

<< 

i 53 - 6 

i A.T. 

.01220 

d 

24.4 

2 A.T. 

.00064 

a 

-32 


In Mexico, where the metric system is used, the ore is weighed 
in kilograms or grammes and an ore is spoken of as carrying 
so many grammes of silver or gold per ton, not ounces per ton. 

Ten grammes of ore are usually taken for crucible assay and 
every 1 /ioo of a milligram of silver, of gold, or of both, equals 
the number of grammes per metric ton. That is, if the silver 
from io grammes of ore weighs .00001 gramme it is equal to 
1 gramme per metric ton and if it weighs .00060 it is equal to 
60 grammes. 

A metric ton equals 1000 kilograms. 


MEMORANDA AS TO WEIGHTS AND VALUES. 


i gramme 
1 ounce av. 


a 


i ” troy 
1 pound av. 


15.432 grams. 

437i “ = 28.34 grammes. 

480 “ =31.11 

7000 “ troy. 


<< 


The ounce and pound in troy and apothecary weights are 
the same. 

One ounce of gold is worth $2o 67 / 100 . 


One dollar gold coin weighs. 25.8 grains troy 

Ten per cent is alloy (copper). 2.58 “ “ 


Gold. 


23.22 




480 grains = 1 oz. troy. 
480 


23.22 


= 20.67 or $ 2 ° 67 /ioo per oz. troy. 


$800 in gold weigh 43.00 oz. troy. 

/. 4X43 or 38.7 oz. troy is gold. 
.'. 1 ounce troy = $2o 67 / 100 . 


“ or $i.oq 


a* ? 

i 











INTRODUCTION 


5 


One silver dollar (by law of U. S. 90 parts silver, 10 parts 
copper) weighs 412J grains. $i2 80 / 100 in silver coin weigh 11 oz. 
troy, or 1 oz. is worth $i 29 /ioo + - $1.29 + Xi6 = $„o 67 /ioo; hence 
the ratio in coinage of 16 to 1. 

A fifty-cent silver piece weighs 12.5 grammes or 192.9 grains. 
1 lb. troy = 5760 grains. 

.\ 2000 fifty-cent pieces weigh 66.9 lbs. troy or 55.1 lbs. av. 


Sterling silver should contain 925 parts of silver in 1000. 
The remainder consists of copper and a small percentage of 
some metal like cadmium, which makes it roll more readily. 
One ton of gold is worth 29,166 0z.Xl2p.67, or $602,861.22. 

One gramme of gold is worth = 66 cents. 

3I-H 

One grain of gold is worth 4 3 /io cents. 


When we speak of gold we often refer to it as so many carats 
fine. In this case we mean parts in 24; that is, if a ring is 22 
carats, it means that 22 parts in 24 are gold, and the other two 
parts alloy of either silver, copper, or both silver and copper. 


An ordinary carat = 205 milligrammes or 3 1 /,, grains troy, i.e., 
151.76 carats = 1 troy oz. Jewelers divide this carat into 4 grains, 
called diamond-grains or carat-grains. 


ASSAY REAGENTS. 

The reagents used in assaying may be divided as follows: 

1 st. Reducing agents. 

2d. Oxidizing agents. 

3d. Desulphurizing agents. 

4th. Sulphurizing agents. 

5th. Fluxes. 

1st. Reducing Agents. — When a metal is separated from 
a state of chemical combination it is said to be “reduced,” and 
the process of separation is termed “reduction.” (Percy.) 



6 


NOTES ON ASSAYING. 


The agent by which the reduction is effected is termed a 
“reducing agent.” (Percy.) 

2PbO+ C = 2Pb+ C 0 2 . 


Here the carbon is the reducing agent. 

PbS+Fe =Pb+FeS. 


In this reaction the iron reduces metallic lead; but we generally 
speak of the iron as a desulphurizing agent. 

A reducing agent is also defined as a substance which is capable 
of taking away oxygen from those compounds with which it is 
combined and which are willing to part with it. 

Chemically, it is defined as a compound or element which 
takes away an acid radical and gives up a basic one. 

The following are the reducing agents most commonly used: 

Charcoal .—From the reaction 2PbO+C =2Pb+C 0 2 , this 


207X2 

should have, if pure, a reducing power of -= 34J grammes 

X 2 

of lead. But as used it is seldom pure, and it has a reducing 
power of only 24 to 28 grammes. 

Argols .—Reducing power of 7 to n, depending upon the 
purity. 

Cream 0j Tartar .—Reducing power about 5 grammes. 
Potassium Cyanide. 

Flour .—Reducing power about 11.9 grammes. 

Starch .— “ “ “ 12 “ 

Rosin. 


2 d. Oxidizing Agents. —These give up oxygen easily. 

Oxygen of the air: 2FeS 2 -f 11O =4S0 2 +Fe 2 0 3 . 

Litharge: PbO + Fe = FeO+Pb. 

2 Pb 0 + S = S 0 2 + 2 Pb. 

Ferric oxide (Fe 2 0 3 ) and Manganese binoxide (Mn 0 2 ).—In 
the presence of carbon these are both reduced to protoxides: 
Fe 2 0 3 -f C = 2FeO+ CO. (See page 72.) 




INTRODUCTION. 


7 


Nitrates oj Potassium and Sodium. —These are most power¬ 
ful oxidizing agents; and if in the presence of sulphides an 
excess is used, H 2 S 0 4 is formed. On sulphides of Ag, Cu, and 
Pb, that is, sulphides not easily oxidized, the nitre, if used in 
exact quantity, will leave the metals pure, and oxidize the sulphur 
to H 2 S 0 4 or S 0 3 or both. On other sulphides it not only forms 
S 0 3 , but also the oxides of the metals: 

4ZnS+ 6KNO3 = 4ZnO + 3K 2 S0 4 4- S 0 2 + 6N. 

A substance is oxidized when oxygen or some other acid ele¬ 
ment or radical is added to it; or when hydrogen or a basic ele¬ 
ment is taken from it. For the determination of the oxidizing 
power, see page 81. 

Alkaline Carbonates. —These owe their oxidizing power to the 
C 0 2 contained and given off upon heating: 

Na 2 C 0 3 = Na 2 0 + C 0 2 . 

3 d. Desulphurizing Agents. 

Oxygen: 2FeS 2 + 11O =Fe 2 0 3 +4S0 2 . 

Charcoal. —Forming sulphide of carbon and reducing sul¬ 
phates to sulphides: FeS0 4 +3C =FeS-f 2CO+C 0 2 . 

Iron: PbS + Fe = FeS+Pb. 

Alkaline Carbonates: 

4 K 2 C 0 3 + 7PbS = 4 Pb+ 3 (K 2 S,PbS) + K 2 S 0 4 +4CO a . 

Litharge: PbS-f- 2PbO = S 0 2 + 3Pb; 

2FeS 2 + nPbO =Fe 2 0 3 -f4S0 2 + nPb, 

or FeS 2 + 5PbO = FeO-f 2S0 2 + sPb. 

Litharge decomposes all sulphides in this way, and in so doing 
lead is reduced and we find that 

1 gramme of pyrite will reduce about 10 grammes of lead. 

1 “ “ blende “ “ “ 9 “ “ “ 

1 “ “ galenite will reduce about 2\ “ “ 11 


s 


NOTES ON ASSAYING. 


Nitre. —This acts under heat as follows: 

2 kno 3 =k 2 o+n 2 o 5 ; n 2 0 5 =2NO+3O. 

6KNO3+ 2 FeS 2 = 3K 2 S0 4 + S 0 3 + Fe 2 0 3 + 6N. 

4 th. Sulphurizing Agents. 

Sul phur. 

Sulphides , such as iron pyrites and galenite. 

5 th. Fluxes. —A flux is something which, if added to a body 
infusible by itself or with difficulty fusible, will cause it to fuse. 
For instance, take some quartz carrying free gold, the amount of 
which we wish to determine. In order to melt the quartz, which 
is acid, we shall have to heat it far above 1064° C., the melting- 
point of gold. Still the gold will not entirely separate. If, how¬ 
ever, we add some Na 2 C 0 3 , a basic flux, to the ground quartz, we 
shall form a fusible silicate of soda, and the gold, owing to its 
high specific gravity, will then separate out easily. 

The sodium carbonate is the flux added: 

Na 2 C 0 3 + Si 0 2 = Na 2 Si 0 3 + C 0 2 , 

and the sodium silicate is the slag formed during the fusion. 

To determine what flux or fluxes to add to any ore or material, 
the student should remember that if the ore is basic, like lime¬ 
stone or iron oxide, it will require a flux which acts as an acid, 
like silica or borax. If the ore is acid, it will require a flux which 
acts as a base, like iron oxide, limestone, or litharge. 

The following are the principal fluxes used in assaying: 


Litharge 

• 

Borax 

Nitre 

Lead 

Borax glass 

Limestone 

Na 2 C 0 3 or NaHC 0 3 

Silica 

Fluorspar 

k 2 co 3 

Argols ) 

Iron oxide 


Charcoal r 
Flour 

KCN 


INTRODUCTION. 


9 


FUSION PRODUCTS. 

In all the fusions he makes, whether by scorification or by cru¬ 
cible, the student should obtain a button of some of the metals, 
a slag, and possibly a matte or a speiss besides. 

Slag .—This is the refuse or waste material from the ore or 
substance worked upon. Slags are either acid or basic. An 
acid slag tends to be glassy and brittle, and when melted can be 
pulled out into long strings like molasses candy. If the slag is 
basic, it is dull and stony-looking; it is tough when cold and can¬ 
not be pulled out into strings when melted or near the chilling- 
point. 

The fusibility of slags varies a great deal. As a rule the 
combination of several substances makes a more fusible mix¬ 
ture or slag than a combination of only two. The slags from 
lead and copper smelters are essentially silicates of iron and 
lime or iron, alumina, and lime; those from an iron furnace are 
silicates of lime, magnesia, and alumina. 

In our assay work this question of fusibility must be con¬ 
stantly borne in mind. We are forming silicates of soda or lead 
or a combination of these with various oxides and borates, and 
although many silicates are very fusible others are quite infusible. 
A fusion may therefore be perfectly liquid in a pot furnace and 
yet thick and full of shots of lead in a muffle furnace, where the 
heat is not so high. 

Slags should be homogeneous and contain no streaks or parti¬ 
cles of substance that are apparently undecomposed. They vary 
in color, depending upon the fluxes used and the ingredients in 
the ore or substance. . A red slag indicates copper oxide (Cu 2 0 ); 
a very light green indicates a ferrous silicate. At times the color 
seems to depend upon the temperature at which the fusion was 
conducted. 

The four following fusions made at one time will serve as an 
example 


IO 


NOTES ON ASSAYING. 


ORE NO. 1551. 

A silicious ore carrying a little iron oxide and a small amount of pyrite. 



1.* 

2 A.f 

2 B.J 

3 -§ 

Ore. 

1 A.T. 

1 A.T. 

1 A.T. 

2 A.T. 

Sodium bicarb., grammes. 

60 

5 ° 

5 ° 

80 

Borax, “ . 

5 

4 

4 

6 

Litharge, “ . 

5 ° 

5 ° 

5 ° 

70 

Argols, “ . 

3 

3 

3 

3 

Salt. 

cover 

cover 

cover 

cover 

Weight of Pb button. 

29 

29 

33 

34 

Weight Ag and Au. 

.02790 

.02805 

.02825 

•05565 

Correction for the Ag in the PbO used. 

.00051 

.00051 

.00051 

.00072 

Weight Au. 

.00275 

.00263 

.00270 

.00526 

Ounces per ton Ag. 

24.64 

24.91 

25.04 

24.83 

Ounces per ton Au. 

2.75 

2.63 

2.70 

2.63 


* Fused 5 5 min. at bright heat. Slag, dark gray, stony, homogeneous, opaque. 

t Fused 55 min. at bright heat. Slag, dark gray, streaked with black, somewhat 
glassy, opaque. 

t Fused 55 min. at much lower heat. Slag, green, stony, homogeneous, translucent on 
edges. 

§ Fused 55 min. at bright heat. Slag, dark green, streaked with black, somewhat 
glassy and translucent on edges. 


The specific gravity of a slag must also be taken into con¬ 
sideration, for if it is too high the metal to be recovered will 
not readily settle out. 

All slags obtained from any work should be kept in the proper 
trays on the iron table, for they will soon destroy the furnace- 
linings if they get mixed with the fuel. 

The student may also meet with the following: 

Matte or Regulus. —These terms have the same meaning. 
The former is generally used in this country, and the latter abroad. 
They are applied to a metallic sulphide, formed by the com¬ 
bination of the metal with sulphur at an elevated temperature. 

Copper matte = 2 Cu+S = Cu 2 S; 

Iron matte = Fe -fS = FeS; 

Lead matte = Pb + S = PbS. 


Speiss or Speise.— This term is applied to a metallic arsenide 
or antimonide formed in smelting operations, so we speak of a 
nickel-cobalt speiss or an iron speiss. 

Examples: 


Nickel Speiss: 


Ni = 45 %; Co =6%; Fe = 9 %; 
As = 36%; S =2%; Cu = i%. 


Iron Speiss = Fe 5 As, containing 21.1 2% As. 




























INTRODUCTION. 


11 


If we use iron in assaying an ore containing arsenic, we often 
CH&T —Speiss. Hard and britu* find as the result of the fusion a lead 
w button and a speiss lying above it thus: 

A high temperature and a small amount of alkaline flux tend 
to the formation of an iron speiss (see assay of gold ores, page 137). 

If metal, speiss, matte, and slag were all 
present in one fusion, they would separate out 
in the order given in the accompanying figure. 

It seems rather doubtful in what condition 
the antimony and arsenic exist in a speiss. 

It is generally true that as the antimony and 
the arsenic increase in amount, the specific 
gravity of the speiss also increases. 



Slag 

Matte 

Speiss 

Metal 


FURNACES AND FUELS. 

In our assay work we make use of two kinds of furnaces, 
the muffle and the crucible furnace. In the latter our assay 
vessels are in direct contact with the fire, while in the former they 
are not. These furnaces may be heated with solid, liquid, or 
gaseous fuel, the choice and use depending partly upon price 
and partly upon locality. 

Crucible fusions can also be made in a muffle, and some fur¬ 
naces are built in combination. 

Our solid fuel consists of charcoal, coke, anthracite, and bitu¬ 
minous coal. Any of these may be used in heating a muffle-furnace, 
the first three heating it by actual contact, the last by its flame 
alone. All, with the exception of the bituminous coal, can be 
used for the crucible-furnace. Gaseous and liquid fuels may be 
used in either furnace. 

In using solid fuel the furnace has to be constantly fed; 
whereas in using gaseous or liquid fuel the supply can be regulated 
and the heat much better adjusted. 

The furnaces themselves may be made of bricks or tiles alone 
and then hooped with iron, or an iron shell may be made of the 
desired shape and size and then lined with fire-brick on the 
inside. These bricks should fit very closely together, and the 
least practical amount of fire-clay mortar should be used. The 





NOTES ON ASSAYING. 


I 2 

brick and tile furnaces, hooped with iron, are certain to crack, 
owing to the constant expansion and contraction. 

REFRACTORIES. 

Fire-clays.—Fire-clays are practically silicates of alumina, 
and are so named on account of their ability to resist high tem¬ 
peratures without softening. They are also called refractory 
clays. Their plasticity depends upon their water of combination: 

2 Al 2 0 3 , 3 Si 0 2 ; Al 2 0 3 , 2 Si 0 2 +H 2 0 . 

Their shrinkage is from to 5 per cent. 

The impurities most commonly found in clays are oxide of 
iron, carbonate of lime, and the alkalies. These all tend to make 
the clay fusible, as they combine with the silica present in the clay. 

The nearer we can have the clay to a simple combination of 
Si 0 2 and A 1 2 0 3 the better it seems to be; and the larger the pro¬ 
portion of Si 0 2 the more refractory it is. 

Fercy gives the following analyses of clays: 



Stourbridge Clay, 
used for Glass-pots. 

Belgium. 

Dowlais, 
South Wales. 

Kaoline. 

Si0 2 . . . . 

. 63.3% 


57 - 12 % 

67.12% 

53 - 7 % 

ai 2 o 3 ... 

. 23.3 % 


29.06% 

21.18% 

44 - 3 % 

CaO. .. . 

. - 73 % 


.04% 

• 32 % 

Trace. 

FeO. . . . 

. 1.8% 


Fe 2 0 3 . 45 % 

1-85% 

■ 9 % 

H 2 0 and 

organic 





matter. 

. 10.30% 


9 - 30 % 






MgO . 70% 

.84% 

Trace 



Alkalies 1 . 14% 

2.02% 

k 2 o 1.2% 


H z O of combination 4 . 82 % Na 2 0 
Hydroscopic water 1 . 39 % 


Some authorities go so far as to say that a small amount of 
alkalies is rather beneficial, as they act as a sort of cement to 
the material. 

Fire-brick.—These are made from refractory clays and vary 
not only in composition but in the texture of material. Some 
bricks are very coarse-grained and some very fine, depending 
upon where they are to be used. They fuse at between 1400° 
and 1700° C. Prof. C. L. Norton of the Institute of Technology 
found that some fused at between 1600° and 1700° C. 








INTRODUCTION. 


13 


FURNACES USED IN THE LABORATORY AT THE 



CRUCIBLE FURNACE 


This will hold six E or F, five G or H or two Ks. 



It will hold 4 E, F, G, or II crucibles, or 2 K or 2 L. 



































































14 


NOTES ON ASSAYING . 


1 


MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 




Crucible-tongs. 



MUFFLE-FURNACE (judson). 

b = stoking-door. In small furnaces sometimes it is at a. 
c = ash-pit. 
d = muffle. 

e => where fuel is charged. 

* See excellent paper by Mr. Edward Keller on Labor-saving Appliances in 
the Works-Laboratory, A. I. M. E., February, 1905. 










































INTRODUCTION. 


1 5 


Crucibles.—Crucibles are made from prepared clays, or from 
suitable mixtures, in either of two ways: 

i st. By moulding upon a potter’s wheel. 

2d. By compressing the clay or mixture into moulds of the 
desired form. 

Crucibles should have the following properties: 

1. They should be infusible. 

2. They should be able to withstand sudden changes of 
temperature. 

3. They should be only slightly acted upon, not only by the 
charge within, but by the ashes from the fuel without. 

4. They should be as much as possible impermeable to the 
substances fused in them, and also to gases. 

The most infusible crucibles are made from clays having a 
high percentage of Si 0 3 and carrying only small amounts of iron 
oxide, lime, and the alkalies. 

Lime, magnesia, and alumina crucibles are made and are 
extremely infusible, but they are used only for special purposes. 

The infusibility, as well as the power to withstand sudden 
changes of temperature, is increased by adding some substance 
like quartz, graphite, coke, and ground flints to the clay. These 
substances neither expand nor contract and they make a sort of 
infusible framework for the rest of the material. Old crucibles 
or old glass-pots, with the vitrified matter carefully chipped off, 
are sometimes used. Crucibles that are dense and close-grained 
are the least acted upon by the fusion, i.e., the material must be 
finely ground and not coarse. This is the reason why a crucible 
like the Beaufay is so much superior to a Hessian. 

Crucibles are tested, as regards their resistance to oxides, by 
fusing litharge (PbO) in them and noting the time it takes the 
litharge to eat through. 

Filling the crucible with water and noticing the time it takes 
to make the crucible moist upon the outside is a test as to its 
permeability to liquids. 

A clay crucible should not be placed directly upon hot coals. 
It will crackle audibly and later on it may crack. Always put 


i6 


NOTES ON ASSAYING. 


some cold L.el upon the fire and then place the crucible or cru¬ 
cibles upon that. 

Berthier gives the following analyses of crucibles: 


Hessian.... 

Si 0 2 . 
Per Cent 

. 70. QO 

Ai 2 o 3 . 

. Per Cent 
24.80 

FC2O3 
Per Cent. 

3.8° 

MgO K 2 0 . 

Per Cent. Per Cent. 

Made of clay and sand. 

English. 

. 71.OO 

23.OO 

4.00 

j 

Used for melting steel. 

St. Etienne. 

. 65.20 

25.OO 

7 20 

( 

Probably clay alone. 
Used for melting steel. 

Bohemian.. 

.. 68.00 

29.OO 

2.20 

• 5 ° 

Used for melting glass. 

Cornish .. . . 

• 72.39 

25-32 

1.07 

.38 1.14 


Beaufay.... 


34-40 

1.00 



Crucibles are 

used both 

in the burnt and the unburnt condition. 


Small crucibles are generally kiln-burnt. The large clay pots 
made at Stourbridge, England, and used largely by brass-founders 
are never burnt, but are slowly dried and then heated very carefully, 
as a graphite crucible would be when placed in the furnace. 

Oftentimes it is necessary to give crucibles a coating of some 
substance that will prevent absorption by the crucible and yet be 
harmless to the fusion. Silver chloride, for instance, will very 
quickly soak into the pores of a crucible. To prevent this, take the 
new crucible and fill it with a boiling saturated solution of borax, 
allow to stand for some minutes and then pour out. Set crucible 
aside to dry. Borax and borax glass may also be melted in the 
crucible, and then swashed around until the inside is glazed over. 
This glaze not only prevents substances soaking into the crucible, 
but acts as a glaze of salt would, and prevents any metallic par¬ 
ticles adhering to the sides of the crucible. 

When a crucible does not crack, is only slightly corroded, 
and is clean from a previous fusion it may be used again. The 
amount of corrosion depends partly upon the character of the 
charge and partly on the temperature of the fusion. With a 
silicious charge an ordinary crucible will be only slightly attacked, 
while if the litharge is high and the charge basic the corrosion 
will be very marked. A basic crucible, on the other hand, will 
be attacked by a silicious charge. 

The size of most crucibles is indicated by letters or numbers, 
stamped on the side or bottom. The larger the number or the 








INTRODUCTION. 


17 


higher the letter in the alphabet, the larger the crucible. Any 
number may be purchased. Original casks of Battersea cru¬ 
cibles contain, of A’s, 1800; of F’s (5" high X3" diam.), 500; of 

G’s (st"X3f"), 4 °°) and of H’s (5|"X3f"), 300. 

The sizes of Hessian crucibles, triangular and round, are 
designated as 3’s, Small 5’s, Centimetres, Large 5’s, Sixes, Eighths, 
Halves, Ones, and Double Extras. 

Covers are sold to fit all sizes of crucibles. 


GRAPHITE CRUCIBLES. 


Graphite is pure carbon when the mineral itself contains no 
Impurities. It may occur massive, earthy, or crystalline, and is 
often found in scales and grains in granite, limestone, and slate. 
The principal sources of supply are Ceylon, Russia, Mexico, and 
Ticonderoga, N. Y. 

Pure graphite neither melts, softens, nor changes in any way 
when heated to very high temperatures, provided oxygen is ex¬ 
cluded. It burns very slowly when heated in the air. 

Percy gives the following as analyses of some samples of 


graphite: 

Sp. Gr. 

Vol. Matter. Carbon. 

Ash. 

( Si 0 2 52.5%; Al 2 O s 28.3%; 

English. . . 

2*34 

1.10% 91-55% 

7 - 35 % ' 

< Fe.O. 12%; CaO and 
( MgO-6% 

English. . . 


86. 7% 

i 3 - 3 % 

Used for pencils. 

Ceylon. ... 


96.1% 

3 - 9 % 

■ 

Russian. . . 

2.17 

• 72 % 94 - 03 % 

5 - 25 % 



As graphite is not plastic, it is mixed with fire-clay and then 
moulded. The proportions are generally one part fire-clay and 
three parts graphite, the clay acting as a frame for the crucibles. 
They withstand extremely high heat and sudden changes of tem¬ 
perature; Oxides, when fused in them, are reduced to the metallic 
state and soon consume the graphite in the crucible. This, 
together with the gradual consumption of the carbon upon the 
outside of the crucible, eventually destroys them. Before using 
they should be kept in a dry place and annealed right side up 
at about 250° to 300° Fah. until free of moisture. When first 



i8 


NOTES ON ASSAYING. 


* 

heated, it is safer to place them in the fire in an inverted position, 
otherwise they are liable to crack and sometimes to explode. 
When red all through they are turned right side up and are 
then ready to receive the charge. In crucible-steel works it is not 
possible to do this owing to the crucible being full when placed 
in the furnace, but there is always a deep layer of unburnt fuel 
beneath them when used. 

The first fusion should be made as rapidly as possible con¬ 
sistent with the safety of the pot, so as to glaze both 
outside and inside by melting the binding material. 

The tongs for lifting them should fit just below 
' _ 

the bulge of the crucible. This avoids any danger 
of undue squeezing and consequently cracking. 

Although the crucibles are all carefully made, there seems to 
be a great difference in those of one lot of the same grade 
and intended for the same purpose. The material to be fused 
in them determines their composition and the manner of treat¬ 
ment during their manufacture. A crucible may stand from one 
to thirty or more fusions, depending upon the manner of their 
handling and the substance fused in them. 

The texture of the graphite and the kind of fire-clay employed 
has a great deal to do with this, as well as the manner of baking 
and firing. (See Iron Age, May 20, 1897, paper by John A. 
Walker.) 

The Jos. Dixon Crucible Co. (mines at Ticonderoga, N. Y., 
and works at Jersey City, N. J.), in making their crucibles, use 
about 50% graphite, 17% sand, and 33% fire-clay (air-dried). 
They have also twenty or more other formulas, according to the 
use to which the crucible is to be put. The fibrous variety of 
graphite is preferred, because its binding properties are greater. 

The graphite must pass a 40-mesh screen; if it is ground too 
fine, the crucible will be too dense; if too coarse, the crucible 
will be too porous. 

The sand must also pass a 40-mesh screen. 

Manufacture.—Formerly the clay was made into a thin paste 
with water and the sand and graphite next mixed in and 
passed two or three times through a pug-mill. The ingredients 



INTRODUCTION. 


*9 


are to-day thoroughly mixed and kneaded in a machine with 
revolving knives and then tempered several weeks in a damp 
place or kept covered with damp cloths. Weighed lumps of 
the tempered material are next kneaded and then moulded 
on a wheel, or else the kneaded dough is put into a plaster- 
of-Paris mould, which it only partly fills, and rammed in hard. 
The mould is now put in an iron holder and set revolving, 
while a plunger is gradually lowered at one side into the mould. 
By its action the dough now gradually rises up to the top of 
the mould and we have a crucible inside a plaster-of-Paris mould. 
This method of manufacture tends to place the graphite flake 
tangentially. The crucibles are allowed to stand in the moulds 
several days, during which time part of their water is absorbed. 
They are then removed, finished, or smoothed on the outside and 
dried for a week or more at 70° to 8o° Fah. Finally they are 
burned in a pottery-kiln, heated by anthracite or a long-flaming 
wood. The temperature is noo° to 1300° Fah., and the flame 
does not touch the crucibles. 

Graphite crucibles are numbered from 00 upward, and up 
to about No. 16 cost so much per crucible; beyond this they 
cost so many cents per number and are supposed to hold 3 lbs. 
of metal per number. 

Scorifiers.—These are the vessels in which the scorification 
process is carried on in the muffle. They are made of refrac¬ 
tory clay which is more finely ground than that used in the manu¬ 



facture of most crucibles. One man, in eight hours, can make 
about 1000 of them, and they would weigh from 150 to 250 lbs. 
The principal foreign makes are the Battersea (English), Beau 
fay, and Freiberg. The forms are either deep or shallow. The 
sizes are 1", ij", ii", 2", 2^", 2j", 2f", 3", 3J", 4", and 5" 
diam., outside measurement. 

Original casks hold 2700 of 2j" diam., 1600 of 3", and 650 
of 4". 









20 


NOTES ON ASSAYING 


Scorifiers may be used more than once, but a # s they are very 
cheap it hardly pays to run the risk of their being eaten through 
the second time they are used and thus losing an assay. If the 
inner surface is rough and much corroded no attempt to use them 
a second time should be made. When scorifying a lead button, 
to diminish it in size or to oxidize the impurities present and to 
slag them off, a small amount of silica (Si 0 2 ) should always be 
added after the lead button has fused and commenced to “drive." 
The PbO combines with this, and by so doing the scoriher is not 
so much attacked and eaten into. 

Cupels.—These are used for the cupellation of lead buttons 
containing silver and gold. They are made from the bones of 
horses or sheep and have the property of absorbing the oxides 
of the base metals, leaving the gold and silver. The bones are 
burned until they are perfectly white, leaving from 60% to 70% 
ash, and are then ground so fine that they will pass a 40- to 60- 
mesh sieve. The bone-ash is then ready for use and consists 
chiefly of calcium phosphate ^CaO^Os), with some calcium 
oxide (CaO). The bones of oxen, according to Bloxam, analyze 
before burning as follows: 


Animal matter. 30.58% 

Calcium phosphate. 57 - 67 % 

“ fluoride... 2.69% 

“ carbonate. 6.99% 

Magnesium “ . 2. 07% 


The ash from the burned bones would then analyze about as 
follows: 

Calcium phosphate 
‘ ‘ fluoride . . 

“ oxide ... 

Magnesium oxide . 


88.00% 
4.10% 
6 - 39/0 
1 -5i/o 


The following will give some idea of the bone-ash on the 
market as passing a 40-mesh sieve: 

1st Barrel. 


2d Barrel. 


On 40-mesh sieve. 

... - 5 % 

■9% 

Through 40 on 60 sieve. . . ., 

- 3 • °°% 

26.2% 

“ 60 “80 “ _ 

- 15.90% \ 

i 9 - 2 % 

“ 80 “ 100 “ _ 

- 27.80% j-96.50% 

\o 

o\ 

00 

M 

0 ) 

“ 100 sieve.. 

.... 52.8% ) 

3 2 - 1 % 


















INTRODUCTION. 


2 I 

The cupels are made by moistening the bone-ash with water 
alone, or with any one of the following solutions: pearl ash, 
borax (i% to 2%) in H 2 0 , sour beer or molasses (1% to 3%) 
in H 2 0 . Some prefer one, some another. It is made just 
moist enough to stick together when pressed in the hand, and 
the cupels must come out of the moulds easily. 

10 to 24 per cent of water will suffice, depending upon the 
freshness and the quality of the bone-ash. A less percentage will 
be required if a binding substance like molasses is added to the 
water. 

It is then ready to compress in the cupel moulds of any desired 
size, either by hand or by machine. The size and shape of the 
cupel varies as well as that of the bowl. The cross-sections, 



actual size, of two used in this laboratory are given. The com¬ 
pressing, if done by hand, is a matter of practice. Some assayers 
prefer to make the bottom layer of a cupel of 40-mesh material 
and then put a finer layer on top, compressing all at once. If 
too much compression is used, the cupels will be too hard, the 
litharge (PbO) will be very slowly absorbed, prolonging the 
cupellation and resulting in the loss of precious metals. 

If too soft, they are fragile and the litharge will be apt to 
carry the precious metals with it into the cupel. 

One kilogram of bone-ash will make from 25 to 32 cupels, in 
which a lead button can be cupelled weighing from 25 to 30 
grammes. 

They should be well dried, preferably air-dried, before using, 
the longer the better, and finally heated to the full temperature 
of the muffle, so that they are red all through , before the lead 
button is dropped into them. If they are moist and contain 
•organic matter, they will “spit ” and throw the melted lead about, 








22 


NOTES ON ASSAYING. 


thus spoiling the assay. Provided the button will go into the 
bowl of the cupel and the cupel is thick enough , it will absorb 
its own weight of PbO. The higher the temperature the more 
the cupels are attacked by the litharge. 

The cracking or checking of cupels may be due to different 
causes. It is more liable to occur in a soft cupel than in a haid 
one and in one which is heavily charged with litharge than in 
one where only a small amount has been absorbed. Too sudden 
heating and the presence of much copper may cause it. 

Cupels can be used only once, and they should never be 
heated to the full temperature of the muffle, taken out, again 
heated and used. The amount of lead absorbed as oxide is from 
| of a gramme to i gramme per minute. The student will save 
time and also danger of cracking the cupels if he has them 
warming on the furnace while he is scorifying. 

Never keep lead buttons in the cupels previous to using them, 
for it injures the surface. The buttons should be placed in the 
cupels only when the cupels are hot and ready for cupellation. 

Muffles. —These hold the scorifiers and cupels. They are 

made of refractory clay and come in various 
sizes and shapes, some with high and some 
with sloping sides. Most of them are 
closed at one end, but some are open at 
both. Some have projections on the inside the whole length of 
the muffle and about half way up. When the muffle is full of 
cupels, pieces of tile or false muffle-bottoms can be placed across 
the muffle on the projections and above the cupels which are 
too hot, thus lowering the temperature and keeping it uniform 
throughout. The size is indicated by letters: 



c 


J is 12" longX6" wideX4" high, outside measurement. 


L “ 15 


// 


u 


X9 


// 


ll 


X6 


tr 


u 


(< 


(( 


The L will weigh about 13 lbs. 

Original casks contain 50 ot the J’s and 25 of the L’s. 

The cost depends upon the size, that of a J being about 80 cts. 
The length of time they last depends partly on the way in 




INTRODUCTION. 


2 3 


which they are supported in the furnace and partly on the care 
with which they are used. 

If the student spills anything in one or a scorifier eats through, 
he should immediately scrape out the substance with a scraper, 
throw in some ground bone-ash and 

scrape it out again. This prevents the - - 

slag or the PbO from eating a hole 

through the muffle. Finally sprinkle in a layer of bone-ash. 

The life of the muffle, as well as that of the furnace, is pro¬ 
longed if the student observes the following precautions. When 
he is through using a furnace, let him shut off all drafts, leave the 
muffle closed and the furnace, whether muffle or crucible, banked 
as much as possible. By so doing all parts will cool down slowly, 
avoiding danger of cracking. 


MORTARS AND LUTES. 

When laying bricks or making repairs about a furnace where 
heat is used, it is always advisable to wet the bricks and the places 
that are to be repaired, previous to applying the mortar. 

Mortars and lutes are always made up dry and thoroughly 
mixed before the requisite amount of water is added. 

Fire-bricks are laid in fire-clay alone or a mixture of § ground 
fire-brick and J fire-clay. The less that is used and the closer 
the bricks are to each other the better. 

Muffles may be set in place with a mixture of 3 parts coarse 
fire-brick (through 12 on 30 sieve), 1 part fire-clay, J part cement 
or f ground fire-brick (through 12-mesh or through 30-mesh 
sieve) and -J- fire-clay with a few pinches of Portland cement. 
This last makes the mixture adhere better and also makes it 
firmer and harder. Another mixture for patching furnaces, 
where the brickwork is broken or torn out, consists of 

7 parts fire-brick (through 12), 

2 ‘ ‘ cement, 

1 part fire-clay. 




24 


NOTES ON ASSAYING. 


If this is put on as dry as it can be and yet stay, so as not to shrink 
away, it will make a patch or joint as solid and as hard as the 
original brick. 

Broken muffles may be made to last many days longer by 
judicious patching with some of the following: 

Where the bottom is almost gone, use a mixture of 

2 parts cement, 
i part ground fire-brick, 
y 3 to y 2 part fire-clay. 

For patching cracks and holes, a mixture of glass, sand, and 
clay, to which a few pinches of litharge have been added, answers 
nicely and, after one good heating, becomes as hard as the muffle. 

In some cases I have used with good results a paste consisting 
of asbestos (short fibre) and silicate of soda. 

Cement used at Idaho Springs, Col., for muffles: 

Fire-clay, 2 parts, 

Litharge, 1 part, 

Bone-ash, 1 “ 

For patching and repairing the walls of crucible-furnaces 
use the first mixture recommended for setting the muffles. Old 
graphite crucibles ground and used alone or mixed with a little 
fire-clay make a splendid mixture which is much used in crucible- 
steel works. 





CHAPTER II. 


SAMPLING. 

Labelling Samples.—Every lot or sample of ore, whether in 
barrels, sacks, boxes, or bottles, should have a name or number 
attached to it. When the sample is received, the first thing to 
be done is to record in a note-book the date received, name, 
number, and any other data connected with it. If the sample 
has no number, one should be given to it to identify it in the future. 

Having noted the number of sample, region produced, date 
received, etc., the next thing to do is to obtain the gross weight. 
If the ore is wet, two samples of from 5 to 20 kilogrammes each 
are taken and the moisture determined. The ore is now dumped 
upon the sampling-floor and the tare of the boxes, sacks, or barrels 
is taken and the net weight of ore obtained. Next the student 
should examine the ore carefully and learn all he possibly can in 
regard to the gangue and the minerals contained therein, for this 
can be done better while the ore is in a coarse condition. 

The sampling comes next, and is done by gradual crushing, 
mixing, and sampling down as performed according to the 
ring-and-cone or Cornish method. All other products coming 
from this original lot should also retain its number. For in¬ 
stance : 

\ 

Sample No. 1420. Lead Ore from Missouri. 

“ 1420-1. Heads from jig A. 

“ 1420-2. Tailings from jig A. 

Any part of the sample not to be used in the test or assay 
should be immediately put in sacks, boxes, or barrels. All 
products, of every description, whether in sacks, hods, pails, or 
boxes, should be labelled in some way, otherwise they are liable to 
be misplaced or thrown away. If to remain about the labora- 

25 


26 


NOTES ON ASSAYING. 


tory for any length of time, they should be covered up and not 
left in open receptacles. 

By observing these few precautions no products can be lost, 
misplaced, or contaminated, as is so often the case. The student 
should bear constantly in mind that if one product is lost, the 
final summing up of the test or run is made impossible. 

In taking up this work I shall give general directions as to the 
methods employed, but shall say nothing in regard to how far 
a lot of ore, of a certain size or richness, can safely be cut down. 
Owing to certain experiments still going on, I am led to believe 
that no rule can be laid down in regard to this, and that each 
lot of ore is a case by itself. 

I do believe, however, that every final sample should be 
crushed through a 120- or 140-mesh sieve at least. The finer 
the sample, the greater the probability of obtaining uniform 
results. If there were only some machine which would do it 
easily and rapidly, and not contaminate the sample, I would 
crush every final sample through a 200-mesh sieve. Many errors 
in assaying and chemical work, as well as non-uniformity in 
results by different analysts on the same sample, are due simply 
to the sample being in a too coarse condition. To send samples 
(other than metallic drillings) which might be passed through 
a 200-mesh screen, but have not been so treated, to different 
assayers and chemists in order to make a comparison of dif¬ 
ferent methods seems to me not only a waste of time, but I 
believe that entirely erroneous conclusions may be drawn from 
the data collected. 

Furthermore , when any experimental work is undertaken on 
a given sample be sure that this sample is so large that , as it 
diminishes in size , all possibility of its changing is eliminated. 

In all sampling work, the student should bear constantly in 
mind two important things: 

1st. That each step in the process must be thoroughly and 
carefully performed. 

2d. That every piece of apparatus or machine must be clean 
and free from all dust and ore previous to its being used. 

By adhering to these rules one ought to obtain a correct 
sample for assay or analysis; by disregarding them one will 


SAMPLING . 


27 


not only obtain an incorrect sample, but will find minerals in the 
sample that were never present in the original ore. 

Ores may be sampled in three ways, each of which has its 
advocates: 

1. By coning and quartering, the Cornish method. 

2. Automatically by machines, especially applicable to sam- 
pling-mills, where the whole stream of ore, after leaving crusher or 
rolls, is taken at given intervals. 

3. Automatically by machines, also applicable to sampling- 
mills, where a part of the stream of ore is taken all the time. 

The first method will be described in these notes. 

We may divide our material into two classes: 

A. Large lots containing over 4000 lbs., also waste-dumps, 
gravel, placer, and similar deposits. 

B. Lots of 4000 lbs. and under. 

Class A.—If the material of this class is in heaps and in a 
five condition, it may be very accurately sampled in the following 
ways: 

1st. By digging cuts or holes into the piles in every direction 
and taking out 50 to 100 lbs. from each place. 

2d. By boring holes into it in various places with a large auger 
2" to 6" in diameter, fitted to a long iron handle. 

The borings should fall upon a piece of canvas, and' all borings 
should be saved. 

In both methods all the portions from the various holes are 
put together, thoroughly mixed, sampled, and quartered down in 
the manner described under Class B. 

If the ore is in coarse large lumps, it is best sampled by means 
of the tape-line. This method is much used on iron ores. A 100- 
or 200-foot measure is taken and dropped over and around the 
ore-pile in different directions. Take a part or the whole of every 
piece of ore upon which each 
foot-mark of the tape rests. 

Any laborer can do this, as 
there is no question of judg¬ 
ment about it. Where a sample is taken by selecting pieces 
here and there all over a pile it is extremely difficult to obtain 
a fair average, for a person’s judgment is influenced, in spite of 




28 


NOTES ON ASSAYING. 


himself, by the appearance of the individual pieces, and this is 
particularly true where he is thoroughly acquainted with the 
character of the ore being sampled. The sample, when obtained, 
is crushed and treated as described under B. 

If the ore comes in cars, which generally hold from 15 to 30 
tons, it is either in a loose condition or in sacks. (If rich, it is 
always in sacks.) If it is in sacks, it is first weighed, and if of low 
grade, i.e., below 100 oz. silver per ton, every fifth or tenth sack is 
taken out and conveyed to the sampling-floor. If it is of high 
grade, every fifth sack is taken. This brings the sample down to 
3000 to 6000 lbs. If the ore is loose in the car, a space is cleared 
in the centre and every fifth or tenth shovelful is set aside for the 
sample. If ore is rich, every third is set aside, the remainder 
going to the ore-bins. This process is repeated until the sample 
representing each car weighs from 3000 to 4000 lbs. If the ore 
is moist, two samples, each weighing at least 5 kilogrammes, 
should be taken at this time to determine the moisture. 

Having our sample of ore, weighing in this case 3000 to 4000 
lbs. and representing the total amount of ore received in the car, 
the whole is treated as in Class B. 

If, in any of the previous samples the pieces of ore are over 


3// 


in size, the whole sample is crushed. 


Before crushing a new lot of ore he sure that the machines 
are perfectly clean and free from the previous lot treated; for unless 
this is done , if that lot of ore was rich , the present sample will 
he worthless. 

Class B.—Having all the ore crushed through a J"-mesh 
screen, spread it in a circle and treat it according to the Cornish 
method, which is here given. 

The ore is first shoveled into a conical heap in the centre of 
the circle of ore, the centre of each shovelful striking the apex 

of the cone and running down 
evenly all round. Spread out 
flat and again draw into a circle, 
or else start a fresh pile and 
keep repeating until it is certain 
that the lot of ore is thoroughly mixed. This conical heap, 8' or 
more in diameter and about 3J' high, is next drawn out into 







SAMPLING . 


2 9 


the form of a truncated cone from 6" to 12" deep and divided 
into quarters. 

Quarters a ' and a" are saved; quarters .v' and go to the ore- 
bins. The quarters saved are mixed as before and shoveled into a 
cone; a shovelful is first taken from a' and then one from a " and 
quartered again. This time quarters x' and x" are saved. Now 
cone and quarter again, mixing by first taking a shovelful from x' 
and then from x". If the sample was originally 4000 lbs., it is 
now 500 lbs. This is crushed in rolls to \ n size and after being 
thoroughly mixed as before it is coned and quartered down to 
250 lbs. It is crushed again in rolls, the whole 250 lbs. passing 
through an 8-mesh sieve (i.e., a sieve with 8 meshes to the linear 
inch. Wire occupies, say, .0280" X 8, or .224"; therefore meshes 
must be .097" each instead of .125"). It is mixed thoroughly again 
and sampled down to 125 lbs. This is crushed in some machine 
so that it will pass through a 12-mesh sieve. The mixing and 
quartering down must now be repeated until the sample weighs 
about 30 lbs. In all this quartering down one must be very 
careful to have the fine portion of the ore belonging to each 
quarter go with it, and not all left each time upon the floor to go 
with other quarters. Always weigh the ore bejore passing it 
through any sieve. 

The 30 or more pounds of ore are put through a 30- or 40-mesh 
sieve and quartered and sampled down 
to 2 to 4 lbs. This is best done, be¬ 
cause it avoids making dust, by using 
a split shovel which is placed in a 
pan and the ore passed over it by 
means of a wide shovel, from aa to bb. 

When the gutters are full the split 
shovel is emptied and this ore kept separate. This is repeated 
until the sample is all passed over the split shovel. 

If what goes between the gutters is saved the first time, 
what fills the gutters is saved the next time, and so on until the 
30 lbs. is reduced to the desired quantity. Or the whole 30 lbs. 
is crushed through a 40-mesh sieve, a sample of 150 to 200 
grammes taken from this, with a broad spatula with high sides, 


a 


2 


a 


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and 

*! 12 gutters 
7 l” high 
j H” wide 

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b w 


























3° 


NOTES ON ASSAYING . 


<z 


and crushed through a ioo-mesh or finer sieve. If the sample 

is at all damp, it is next dried at ioo° C. 
: — * for f of an hour, weighed, and the whole 
amount crushed upon a grinding-plate or 
bucking-board and passed through a 100-, 120-, or 140-mesh sieve. 

Small hand samples and specimens, weighing 600 grammes 
or under, should be crushed and all passed through the fine sieve. 

The bucking-board must be perjectly clean before it is used. 
If any residue or particles of gold or silver are left on this sieve, 
they are weighed, wrapped in C.P. lead-foil, and cupelled. The 
resulting button is weighed and parted for gold. 

If the particles are suspected of being gold alone, the residue is 
wrapped in C.P. lead, a piece of C.P. silver added, and the 
whole cupelled and then parted for gold. 

These weights and the weight of the ore before passing it 
through the sieve being known, the number of ounces per ton 
can be calculated and reported as so much free gold or silver, and 
they can be added to the assay of the ore passing through the 
100-, 120-, or 140-mesh sieve. (See Calculation of Pellets.) This 
question of metallic particles applies as well to the other sieves 
used, and the finer the sieve the greater the care to be observed 
in regard to these particles. The method of attempting to force 
free gold or other metallic particles through a sieve, by con¬ 
tinually grinding them with some of the ore already pulverized, 
is extremely bad practice. It consumes a large amount of time 
and if the particles are coarse they cannot be forced through 
a fine sieve. Furthermore it coats the machines and bucking 
board with a film of the metal which it is difficult to remove 
and it makes the fine ore, which otherwise might be quite even 
and uniform, very uneven. 

The fine portion of the sample, i.e., the part which has passed 
through the ioo-mesh or finer sieve, is put 
upon a sheet of glazed paper, rubber, or oil¬ 
cloth and thoroughly rolled over and over 
again for 100 times at least. It is then 
spread out thinly and divided into squares 
as in the annexed figure. A portion is taken 




























SAMPLING . 


3 1 


with a spatula from each and every square, representing a sec¬ 
tion from the top of the ore to the oilcloth ( that is, do not take 
the upper surface oj the ore alone). 

Fill from one to five eight-ounce bottles. The contents of 
these bottles should be identical and should represent a fair 
average of the original ore, whether it was a carload lot or a sack 
of ore. 

In all the previous work every precaution should be taken 
against making dust and losing the fine ore. 

The ore is now ready to be assayed, for passing an ore through 
a 100-, 120-, or 140-mesh sieve generally makes it sufficiently fine 
for assay purposes. In some special cases it must be pulverized 
even more finely. In weighing out the ore, always empty the entire 
ore out oj the bottle or its receptacle and thoroughly mix it by rolling 
it over and over at least 100 times. 

This is particularly necessary if the bottle has stood any length 
of time, for some ores seem to stratify quite readily on standing. 
The coarser the ore and the greater the difference in the specific 
gravity between the heaviest and the lightest particles the more 
likely is this to occur. 

Weigh out the ore just as carefully as you can upon the pulp- 
balances and assay in the usual manner for whatever element you 
are determining. 

Report the results for silver and gold in ounces troy per 
2000 lbs. of ore av. Metals such as lead, copper, tin, etc., are 
reported in percentages. If the ore carries free, i.e., native, gold 
or silver, it may also be reported as follows: 


Gold (free), i.e., on sieve..oz. per ton 

Gold in fine ore, through sieve..“ 


Total..“ 

At $20.67 P er oz * (U. S. standard value) =$ - 


a 


u 


Silver is reported in the same way as gold, but the vaiue is 
figured at the market rate, which of course varies from time to 
time. 

It has been said that all machines should be thoroughly 






3 2 


NOTES ON ASSAYING. 


clean before any sample of ore is passed through them, and the 
following example will show why this is so essential: 

About 6 lbs. of ore carrying free gold and running 550 oz. 
to the ton was crushed in one of the ordinary rotary sample-mills. 
123.6 grammes of fine quartz sand was then run through the 
machine. This sand, previous to passing through, assayed .04 oz. 
in gold; after passing through it ran .78 oz. Two more lots of 
quartz carrying .03 oz. of gold, and weighing 480 and 555 
grammes respectively, were passed through the machine, and the 
last lot assayed .15 oz. of gold. 

What applies to a machine also applies to the bucking-board, 
which should always be thoroughly cleaned by crushing at least 
two lots of clean sand upon it; and if an unusually rich sample 
has been pulverized upon it, even a more thorough cleaning 
should be given to it. 

Too much care cannot be given to this part of the assay work, 
for I have known many inaccurate assays to result from lack of 
it, and several instances where worthless ore was reported as 
carrying values. 

ORES CARRYING METALLIC PARTICLES. 

Gold and silver ores carrying metallic particles, and others, 
such as the copper ore of Lake Superior, will leave, when crushed 
and passed through a sieve, more or less of the metal upon it, 
the amount depending on the coarseness of the particles and the 
size of the mesh of the sieve. 

The coarser the particles are and the more numerous, the 
more difficult it is to obtain a sample which represents the original 
ore, therefore our aim should be to remove these particles at 
every opportunity. By so doing, although we may not prevent 
all the metallic particles passing through the sieve into our final 
sample, the fineness of these particles makes it more likely that 
we will obtain uniform assays than if the particles were coarser 
and more numerous. 

When these pellets are met with, students always seem to have 


SAMPLING. 


33 


difficulty in calculating their results. If they bear the following 
in mind, this difficulty ought, in great part, to disappear. 

Weigh the original sample. 

Weigh the ore before passing it through a sieve. 

Weigh and determine the amount of metal on each sieve, 
and know from how much ore it has come. 

Weigh, assay, or analyze the fine ore passing through the last 
sieve. 

Calculate the total amount of metal in the entire sample 
of ore. 

From this result and the weight of the original sample calcu¬ 
late the per cent of metal or the ounces per ton. 

The following will serve as examples, and it should be noticed 
that the amount of the material left upon the sieve or sieves, 
the richness of this material, and the percentage which it is of 
the whole sample has everything to do with the final results. 
They may be higher or lower than the analysis of the finest ore 
passing through the final sieve. 

Example I. A sample of lead dross weighs ioo grammes and 
is crushed through a 20-mesh sieve. 

Lead pellets on sieve weigh 40 grammes. 

Material through the sieve weighs 59 grammes (assays 10% Pb). 

Loss in grinding, 1 gramme. 

The total lead = 40 grammes on the sieve. 

Lead in fine material, supposing the gramme lost to assay 
the same as the 59 grammes passing through the sieve = 6 grammes. 
Total = 46 grammes. 

— =46% of lead in the dross. 

100 

Example II. A sample of ore, carrying metallic copper, 
weighs 94 grammes and is crushed through a 120-mesh sieve. 

Residue on the sieve weighs 10 grammes and yields on analysis 
9.32 grammes of copper. The fine ore (84 grammes) through 
the sieve analyzes 20.38% copper. 



34 


NOTES ON ASSAYING. 


84X20.38% contain 17.12 grammes copper 
Pellets contain 9.32 “ 


26.44 

94 


Total = 26.44 


u 


u 


= 28.13% copper in the ore. 


Example III. Sample of concentrates, carrying free gold, 
weighs 35 grammes and is crushed through a 120-mesh sieve. 
The residue on the sieve weighs 2 grammes and consists of pieces 
of iron and free gold. On cupellation and parting, it yields 
.00015 grammes of gold. 

The fine concentrates through the 120-mesh sieve (33 grammes) 
assay 4.09 oz. per ton. 

a rj. Ore through Gold in 

1 120 Sieve. i A. T. 

29.16 : 33 :: .00409 : a = .00463 

Gold found in residue on the sieve = .00015 


Total gold in sample = .00478 
35 : 29.16 : : .00478 : x = . 00398. 


Concentrates assay 3.98 oz. gold per ton of 2000 lbs. This makes 
the final result lower than that of the fine concentrates passing 
through the sieve, and is due to the large amount of material left 
on the sieve and its being poorer in gold than the remainder of 
the concentrates. It also shows that it would be incorrect to 
find the ounces of gold in the two grammes of residue and add 
this result to the ounces (4.09 oz.) found in the fine concentrates. 

Example IV. A sample of ore, carrying free gold, weighs 57 
grammes and is crushed through a 120-mesh sieve. The residue 
on the sieve consists of pieces of mica and free gold and weighs 10 
grammes. On scorifying, cupelling, and parting it yields .0630 
grammes of gold. The fine ore through the sieve assays 2.62 oz. 
per ton. 

29.16 : 47 :: .00262 : a = .00422 
Gold on sieve = .0630 


Total gold in ore = .06722 grammes. 
•\ 57 : 29.16 :: .06722 : x=. 03438. 






SAMPLING. 


35 


Ore assays 34.38 oz. gold per 'ton of 2000 lbs. In this 
case the material left upon the sieve is very much richer than 
the fine material passing through the sieve, hence the final result 
is higher than the assay of the fine material. In this example it 
can be readily seen how absurd it would be to find the ounces 
which the residue on the sieve assays and then add the result to the 
assay of the fine ore passing through the sieve. 

The examples given are not made up, but are some which 
have been met with in actual work. In all of them the material 
which is lost in grinding and sampling is assumed to assay the 
same as the fine material passing through the last sieve. We 
really know nothing about this lost ore, whether it is richer or 
poorer than the ore that is assayed. It may be richer, it may be 
poorer, but it has got to be taken account of, and I think it fair 
to consider the ore lost to assay the same as the fine ore. 

Where pellets are left on a sieve with other matter it is always 
better to treat the whole material. For instance, if free gold and 
metallic iron are in the residue, it is not always safe to remove the 
iron with a magnet, for some of the gold may have been pressed 
hard on to the iron, and when the iron is removed the gold goes 
with it. 


CONCENTRATION BY PANNING OR VANNING. 

This is to determine the percentage of concentrates in an ore 
or to separate any material of value from that which has no 
value, i.e., waste or tailings. 

Take the ore you sampled (through 30- or 40-mesh sieve), or 
else take 20 grammes of pyrite (sp. gr. 4.95 to 5.10) and 150 
grammes of quartz (sp. gr. 2.65), and recover the concentrates 
from the ore or the pyrite from the quartz. 

First , record in your note-book all the data upon the bottles , bags, 
or samples given you. 

Second, order from the supply-room two gold-pans and one 
six-inch evaporating-dish. 

If the ore you sampled is rich in sulphides, weigh out 100 


NOTES ON ASSAYING . 


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43 


C/3 

C /3 


'E, 


cj 


g 


O 


G 


4 -» 


C /3 


"O 


G 


o 




C /3 

o 


X) 

> * 


Dh 


O 


a, 


biD 






C /3 


cd 


C /3 

• ^ 


O 


C /3 

43 


H 


> 




_43 




’c /3 




t-i 




43 




^5 




-M 




O 




43 




45 




- 4-1 





S3 

o 


G" 

CO 


T'. 

G" 

CO 

O 

o 


CO 

°o 

VO 


so 

sO 

M 

Os 


o 

o 

vo 

Os 
















38 


NOTES ON ASSAYING. 


grammes of it on the flux-balance; if poor, 200 grammes. If you 
are working upon the pyrite and quartz, weigh the pyrite on the 
pulp-balance and the quartz on the flux-balance. Put the ore 
into one gold-pan (or the pyrite with the quartz over it), moisten 
very thoroughly to make any float material sink, fill the pan nearly 
full of water, allow to stand ten or fifteen minutes, and then 
pan the material from this pan into the other. The panning 
is done by having the ore covered with water and the contents 
of the pan thoroughly liquid. While shaking well, to allow the 
heavy material and concentrates to settle out, give the pan a 
rotary or side motion and every now and then throw some of the 
gangue or lighter material over the edge. Continue to do this until 
no more gangue can be removed without washing over some of 
the concentrates. Transfer concentrates to the evaporating-dish 
and save. Pan the ore over again, and repeat until you can obtain 
no more concentrates or pyrite. Do not throw away any water 
or waste ore. Allow the concentrates in the 6" evaporator to 
settle for some time and then carefully decant the H2O. Dry 
carefully and as quickly as possible, but do not make so hot 
that the concentrates will begin to roast, and weigh on the pulp- 
balance. Allow tailings or waste to stand some time, decant 
off H 2 0 , transfer to an agate pan or bowl, dry, and weigh on 
flux-balance. The iron gold-pans must not he heated or the 
material left in them , for they rust and are ruined. Clean them 
dry them , and return to the supply-room at the end of the exercise. 

Make your report as follows: 

No. of ore 1360. Amount taken =100 grammes. 

Concentrates (FeS 2 , PbS, and ZnS). 10 grammes, 10% 

Tailings. 86 

Slimes or ore lost in process. 4 

100 


86 % 

4 % 


100% 






CHAPTER III. 


ASSAY OF ORES FOR SILVER. 

The assay of ores for silver is taken up separately from the 
assay for gold, because my experience has been that a beginner 
can, in this way, learn the methods of both assays much more 
easily. By taking up the silver first, he has one thing in mind, 
not two; he becomes familiar with the apparatus employed, the 
chief methods used, the making up of different charges, and the 
metal to be determined is present in fairly large amount. 

After this he is much better able to take up the assay of ores 
wherein the metal to be determined is present in much smaller 
amount and which necessarily requires more care in treatment. 

Silver fuses at 961.5 0 C. or, according to Berthelot, at 962° C. 
(1898). Atomic weight = 107.9. Sp. gr. = 10.5. 

Ores containing silver may be assayed by fire in either of two 
ways: 

First. By the scorification method. 

Second. By the crucible method. 

In either case the object is: 

1st. To add some flux or fluxes to the ore to combine with 
the gangue and impurities, leaving a slag free from precious 
metals. 

2d. To mix some granulated lead or litharge with the ore, by 
means of which the silver, together with the gold, is collected and 
alloyed with the lead. 

3d. To separate the lead from the silver and gold by means of 
cupellation. 

The silver in the ores may be native or may occur as real 

silver ores, i.e., ores with a definite composition, as cerargyrite 

39 


40 


NOTES ON ASSAYING. 


(AgCl) 75.26% silver, argentite (Ag 2 S) 87.1% silver, etc., but 
in the majority of ores it is derived from argentiferous minerals, 
such as galenite, blende, pyrite, cerussite, etc., occurring in some 
gangue as quartz, limestone, porphyry, slate, granite, etc. 

SCORIFICATION METHOD. 

(The Greek word crKopia means slag, i.e., this is a slag¬ 
making process.) This is the more simple of the two methods, 
and although adapted to most ores is especially so to copper 
mattes, copper ores, ores rich in antimony, zinc residues, and 
similar substances. It is an oxidizing process and these impurities 
(Cu, Sb, and Zn) are oxidized directly by the air, or by the litharge 
formed in the process, and volatilize as oxides, or else pass into 
the slag or waste material. 

In the crucible process Cu, Sb, and Zn, which we wish to 
eliminate, are liable to be reduced and to pass into the lead button, 
and have to be subsequently removed. 

Ordinary Ores.—The process is carried out as follows: Take 
the sample (if it is in a bottle or receptacle, empty the whole of it 
out) and roll it on oilcloth or glazed paper at least 100 times. 
This is especially necessary where a sample has just been ground 
or has stood; by standing, samples are apt to layer or stratify, 
and the longer an ore has stood and the greater the difference 
in the specific gravity of the constituents of the sample, the more 
thorough this mixing should be. If two substances of different 
specific gravity and color, like soda and litharge, are put on a 
paper, it will be found that a very thorough mixing is necessary 
before the whole mass becomes perfectly homogeneous. 

Weigh out very carefully two portions of ore, of -* 0 - A. T. 
each, on the pulp-balances. The ore should be fine enough to 
pass through a 100-mesh sieve at least, i.e., 100 meshes to the 
linear inch. Place the weighed amounts of ore in two scorifiers 
(2J" and 2}" in diameter respectively), carefully brushing out 
the scale-pan each time. On the flux-balances weigh carefully 
35 grammes of granulated lead and mix approximately one half 


ASSAY OF ORES FOR SILVER. 


4i 


of this with the ore in the 2J" scorifier, and place the other half on 
top. Then weigh 45 grammes of lead and treat the other por¬ 
tion of the ore with it in the same way. Place a pinch of borax 
glass (about 1 gramme) on the top of the contents of each scori¬ 
fier. The scorifiers are now ready for the muffle, which has been 



Scorifier-tongs. 


previously heated and which should now be very hot. By means 
of the scorifier-tongs place them in the muffle and close the door. 

Fusion Period. —The door of the muffle should be kept closed 
some time , to allow the contents of scorifiers to become thoroughly 
fused. Oftentimes the scorifiers spit owing to the air being 
admitted too soon, which occasions too violent oxidation. 

Roasting Period .—Open the door of the muffle and admit a 
full supply of air. The ore and lead have now either become per¬ 
fectly liquid or else small patches of ore are seen on the lead bath. 

In either case a full supply of air is necessary to roast and 
oxidize the impurities in the ore and also to oxidize the lead to 
litharge (PbO). This and the air are our decomposing agents; 
by means of them volatile substances like As and Sb are oxi¬ 
dized to As 2 O s and Sb 2 0 3 , and either volatilize as such or else pass 
into the slag. 

Scorification Period. —Metals like copper and zinc partly slag 
off and partly go into the lead button. Any sulphur in com¬ 
bination with these and other metals is oxidized to S0 2 and 
volatilizes. 

The vapor arising from the assays will often indicate the 
character of the ore. Sulphur gives clear gray vapor; arsenic, 
grayish white; and antimony, reddish. Zinc vapor is blackish 
and the zinc burns with a bright white flame. 

Ij the contents oj the scorifiers do not become thoroughly liquid 
and do not show a good clear lead surjace } the assays need either 
more heat , more borax glass , or more lead. 

If the assay is in a satisfactory condition during the roasting 






42 


NOTES ON ASSAYING. 


and scorification periods the litharge formed partly combines 
with the gangue of the ore and partly with the material of the 
Bcorifier itself. The slag thus formed goes to the circumference 
of the scorifier, leaving a lead surface or eye exposed. (If the 
muffle is too cold, the litharge formed will make a film over the 
eye of lead and the scorification stops.) 

The slag gradually increases, the lead eye grow r s smaller and 
smaller, and finally the slag closes over and completely covers the 
lead. 

The scorification period is now ended. The ore ought to be 
completely decomposed and the slag quite free from or very low 
in silver, and the remainder of the silver and gold in the ore should 
be alloyed with the lead. 

Liquefaction Period. —Close the door of the muffle and increase 
the heat for a few minutes to make the contents of the scorifiers. 

thoroughly liquid and to insure a clean pour. 
Pour the contents into a mould which has been 
coated with chalk, iron oxide, or oil, previously 
warmed and dried. The inside surface of the 
scorifiers should be clean and show no lumps 
of ore or undecomposed material. 

When cold, break the lead from the slag, which should be 
perfectly free from any small lead buttons; these are most likely 
to be on the circumference of the slag. Hammer the lead into 
the form of a cube and weigh on the flux-balance. If this lead is 
soft and malleable, it is ready for cupellation. (See Cupellation, 
page 55.) If it is hard or brittle, it may contain impurities which 
must be removed by rescorifying with an additional amount of 
granulated lead. (See Rescorifying Buttons.) 

Brittle buttons may be due to Cu, As, Sb, Zn, S, PbO, or a rich 
alloy of Pb and Ag or Pb and Au. Hard buttons may be due to 
Cu, Sb, or a rich alloy. 



The essentials, in this scorification process, are: 
Heat. 

Granulated Lead. 

Air. 




ASSAY OF ORES FOR SILVER. 


43 


The accessories are: 

Borax glass and silica. 

The variables are: 

Borax glass. 

Silica. 

Granulated Lead. 

Temperature. 

Size, depth, and diameter of scorifier. 

The following are some of the reactions which probably take 
place in assaying, for example, an ore consisting of PbS+Ag 2 S 
+ FeS 2 and Sb 2 S 3 in a silicious (Si 0 2 ) gangue. 

When contents of scorifier are liquid and air is admitted we 
have 

Pb+O =PbO; 

PbS+ 2 Pb 0 = 3 Pb+S 0 2 ; 

Ag 2 S+ 2PbO = 2PbAg (alloy) + S 0 2 ; 

FeS 2 + 5PbO = 5Pb+ FeO+ 2S0 2 ; 

Sb 2 S 3 +9PbO = 9Pb+Sb 2 0 3 + 3 S 0 2 ; 

Sb 2 S 3 + 9O = Sb 2 0 3 + 3S0 2 ; 

Sb 2 S 3 +6PbO = Sb 2 Pb 6 -f 3 S 0 2 ; 

Si0 2 +2Pb0 = 2Pb0,Si0 2 or one of the lead silicates. 
Thus we shall have: 

Lead acting as a collector of the precious metals, contami¬ 
nated with a little antimony. 

Sb 2 0 3 and S 0 2 as volatile substances. 

Lead silicate, FeO and Sb 2 0 3 , as slag-forming material. 

If Cu 2 S were present in an ore, we should probably have the 
following reactions: 

2Cu 2 S+ 7PbO — 2 Cu 0+ 2 S0 2 + 7Pb+ Cu 2 0 . 

Part of the Cu 2 0 would go into the slag and part of it would 
be reduced, and this copper would pass into the lead button: 

2 Cu 2 0 + Cu 2 S = 6Cu+ S 0 2 . 

We might also have 


2 CuO + Cu 2 S = 4CU+ S 0 2 . 


44 


NOTES ON ASSAYING . 


This copper would make the lead button brittle and necessi¬ 
tate one or more extra scorifications. 

If ZnS were present, we probably should have the following: 

ZnS + 3PbO = ZnO + 3Pb + S 0 2 ; 

ZnS+30 = Zn 0 +S 0 2 . 

Some of the ZnO will volatilize and some will go into the slag or 
form part of it. A little will no doubt be reduced and pass into 
the lead button: 

2ZnO + ZnS = 3Zn+ S 0 2 . 

In order to avoid a heavy loss of silver and gold in the slag no 
oxysulphides should be present there.- 

Some ores require no addition of borax glass, others require 
a large amount. A little in every assay does no harm, but too 
large an amount may cover over the lead bejore the ore is decom¬ 
posed , thus spoiling the assay. 

Borax glass acts as an acid flux and is especially useful in dis¬ 
solving and combining : with the oxides formed during scorification. 

The effect may be shown in assaying some antimonial silver 
ore. (Mitchell’s Assaying.) 

Ore ~ A.T. Lead 24 grammes: slag carried considerable 

. silver. 

“ “ “ “ 50 “ slag still carried silver. 

“ “ “ “ 50 “ and 3 grammes of borax 

glass: slag was free from 
silver. 

Ores containing much lime, zinc, and arsenic also require a 
large amount. 

Heavily sulphuretted ores, concentrates, or ores deficient in 
gangue require the addition of silica to take the place of the gangue. 
If the gangue in most ores is 50% to 90% and ~ A.T. of con¬ 
centrates with little or no gangue is used, then it will be necessary 
to add from 1 to ij grammes of Si 0 2 . If the inner surface of 
the scorifier is rough and much corroded, this is a sure indication 
that the ore is deficient in gangue and that silica is needed. 

As a general thing, the ordinary run of ores requires only 


ASSAY OF ORES FOR SILVER . 


45 


from 35 to 45 grammes of granulated lead to ^ A.T. of ore and 


will be decomposed by that amount of lead with the addition of 

some borax glass. 

The following ores, however, 

require a much 

larger amount : 

Ore. 

Lead required. 
Grammes. 

Heat. 

Borax Glass. Fine SiO. 
Grammes. Grammes. 

Antimonial ores. 


50-6° 

* 

High 

3 to 7 

Arsenical . 

u 

60 


U 

< t 

Cobalt and nickel ores,. 

a 

• • 

6 5 


u 

U 

Copper matte. 

< < 

70-90 

u 

Very low 

1 to q 1 

Copper ores. 

i < 

O' 

0 

1 

^4 

0 

<u 

bD 

i i 

“ i 

Galena. 

u 

40-45 


Medium 

2 to 3 

Iron speiss. 


70 

0 

High 

5 to 7 1 

Jeweller’s sweeps. 

a 

40-45 


Medium 

3 to 5 

Lead matte. 


50 

0 

Low 

1 to 2 1 

Lead speiss. 

a 

60 

0 

in 

High 

“ 1 

Manganiferous ores .... 

11 

5 ° 

V 

CO 

Medium 

2 to 3 

Pyrite (FeS 2 ). 

<< 

45-50 


U 

U 

Stanniferous ores. 

u 

60-70 


High 

“ 1 

Zinc ores. 

<< 

60 

d 

a 

3 to 5 I 


Many of the above, especially the copper and nickel ores, will 
require two or more scorifications before the lead button is fit to 
cupel. 

According to Karsten, it takes 10} parts of lead to carry off 
1 part of copper completely. That is, 1.0500 grammes of lead 
would be required to completely remove .1000 grammes of copper. 

In assaying any ore it is better for the student to use differ¬ 
ent amounts of lead. For instance, if he takes three portions of 
the same ore, he can use 40, 45, and 50 grammes of lead to each 
F A.T. portion of the ore. If his results check, after making 
his corrections for the silver in the lead used, so much the better. 
If the highest lead gives the highest result or if the silver obtained 
increases with the lead used, it will be advisable to try two other 
portions with still higher lead, for the ore evidently requires it. 

Always weigh the lead carefully, as it generally contains silver, 
and a correction has to be made for it later on when the results 
are calculated. 

The weight of the button after scorification depends upon how 
much gangue the ore contained, the amount of lead, borax glass, 
and Si02 used,the diameter and the depth of the scorifier,and lastly 
















4 6 


NOTES ON ASSAYING . 


its position in the muffle. A A.T. of one ore and 45 grammes of 
lead in a 3" scorifier gave a resulting lead button weighing 3.5 
grammes when the scorifier was in the front part of the muffle and 
5.8 grammes when in the back part. 

The same ore and amount of lead scorified at the same time 
in a scorifier 2}" diam. and ij" deep gave a button in the front 
of the muffle weighing 18.5 grammes and in the back of the muffle 
20 grammes. 

From this it is evident that a 3" scorifier is too broad where 
only 45 grammes of lead are used, for it is not always safe to have 
the resulting lead button weigh less than 10 or 12 grammes. It 
is also evident that it is possible to scorify the lead almost com¬ 
pletely away. 

Always notice the color of the scorifiers after pouring, for the 
silicates and oxides of the different metals give very character¬ 
istic colors and hints as to the method of conducting the crucible 
assay, if the ore can be assayed in that way. 

Copper colors the scorifier dark green to light green. If 
much iron is in the ore, or if it is a matte, this color will be partly 
obscured by the black of the iron oxide in the first scorification. 
The scorifier will not necessarily be green if the ore carries only 
9 to 12 per cent of copper and yV A.T. is used. 

Iron colors the scorifier black to dark brown. Peroxide of 
iron is yellow or orange. 

Cobalt makes the scorifier blue and gives a blue slag. 

Nickel “ “ “ black. 

Lead “ “ “ lemon-yellow to very light yellow. 

Manganese colors the scorifier brownish black to pink. 

Arsenic and Antimony, if present in large amount, will leave 
crusts on the inner surface of scorifier on a line where the slag 
came even if much borax glass is used. 

Ij a scorifier is colored very dark green, it indicates directly to 
the student that the lead button must contain copper, and that the 
button must be rescorified in a new scorifier with or without an 
addition oj lead in order to slag and remove this impurity {copper). 

Rescorifying Buttons.—Buttons weighing over 30 grammes 
had better be scorified, whether they contain impurities or not, as 
they are rather large to cupel. Place the scorifier in the muffle, 



ASSAY OF ORES FOR SILVER. 


47 


heat to scorifying temperature (to prevent possible spitting ), and 
then drop in the lead button; after it has been driving a short time 
add a little fine Si 0 2 in order to save the scorifier. Pb +0 = PbO 
and 2PbO +Si 0 2 = 2Pb0,Si0 2 . The slag will consist of silicate 
and oxide of lead, oxides of the impurities in the lead, and oxides 
that have come from the scorifier. When impurities like copper 
are present sufficient granulated lead is added to bring the total 
weight of lead in the scorifier up to 60 grammes. The copper is 
oxidized and slagged by the PbO and Si 0 2 , and a low tempera¬ 
ture is most suitable for it. Sometimes a button requires three 
or more scorifications before it is sufficiently soft or pure to cupel. 
If cupelled before this, the button would freeze and the assay be 
worthless owing to the copper. 

Keep account of all the granulated lead used in case there is a 
correction to be made for its silver contents. 

A large lead button, containing no impurity, which has been 
scorified to diminish it in size is often brittle. This is due to 
PbO, formed during the second scorification, which the lead has 
taken up. 

Bismuth* is the only metal that could be used to take the 
place of lead in the scorification process. It has many of the 
characteristics of lead, but is much more expensive. Owing to 
its low melting-point, buttons from scorification do not chill 
easily and much time must be allowed for them to cool in. In 
cupellation, the silver losses are higher than when lead is used, 
which is due to absorption. On this account the cupels should 
be made of very fine bone-ash and be very hard. The “blick” 
is not as distinct as when lead is used and the silver beads are 
often irregular, instead of being round and smooth, and are likely 
to contain bismuth. The color of the cupel is very noticeable, 
being bright orange-yellow or colored with alternate rings of 
orange-yellow and greenish black. 

The following are some cupellation experiments. 


* Bismuth in cupellation, by Cliaudet. Ann. Chim. et de Phys. (3), vol. 15, 
P- 55 - 


I 





4 8 


NOTES ON ASSAYING. 


Number. 

Bismuth, C.P., grammes... 
Silver, “ “ .... 

Lead, “ “ .... 

Copper, “ “ .... 

Time of cupellation, minutes 
Silver lost in cupelling, per 

cent. 

Silver recovered from cupel, 

per cent. 

Silver assumed to be volatil¬ 
ized, per cent. 


I 

10.0000 

.2012 

2 

3 

5.00000 
.20008 
5.00000 

4 

.20008 

10.00000 

.20030 

10.00000 


19 

14 

17 

15 

10.20 

2.66 

9.91 

2.14 

00 

Ui 

4- 


6.79 

1.88 

1.86 


3.12 

.26 


5 

10.00000 
.20030 
3.20000 
.20000 

17 

6.23 

head 

contained 

some 

copper 


Nos. i, 2, and 3 were cupelled at one time and 4 and 5 at 
another. The high losses in volatilization shown in Nos. 1 and 3 
are doubtless due to the presence of bismuth in the silver beads 
obtained from the first cupellation. 

Spitting of Ores during Scorification.—This often takes place, 
but only during the first five or ten minutes after the scorifiers 
have been placed in the muffle. 

According to my observation, it may be due to the following 
causes: 

1. Dampness of the scorifiers. 

2. Imperfect mixing of the charge. 

3. Admittance of air into the muffle too soon. 

4. Insufficient heat when scorifiers are placed in the muffle. 

5. Too deep a scorifier in proportion to the charge. 

6. Character of the ore itself. 

Sometimes when a lead button is rescorified, either to dimin¬ 
ish it in size or to remove impurities, it will spit. If the scorifier 
was heated before the lead was put into it, the spitting would not 
take place, which seems to indicate that something was wrong 
with the scorifier itself. 

Imperfect mixing of the charge, which is a cause of spitting 
at times, seems also to be one of the causes of spitting in cases 
3, 4, and 5. 

If ore is left at the bottom of a scorifier, it does not fuse or get 
pasty until after the lead has melted above it. As this ore becomes 
hotter it swells and gives off C 0 2 or other gases, and as it swells 
and the gas escapes it throws up particles of lead, which may or 
may not fall back into the scorifier. 


I 





















ASSAY OF ORES FOR SILVER. 


49 


Admitting air into the muffle too soon will certainly cause 
spitting in many cases, especially in the case of material carrying 
much zinc, such as the precipitates from the zinc boxes in the 
cyanide process for treating gold ores. 

Some ores will not spit under any circumstances, but if an ore 
tends to be rather infusible, giving off much gas while the heat is 
not high enough at first, the lead will melt first while the ore is 
still pasty either beneath or all through the charge. In such 
cases scorifiers will often spit and soon afterwards a succession 
of small pieces of ore will rise to the surface and be oxidized by 
the air, litharge, or both, passing off to the circumference and 
disappearing in the slag already formed. 

A deep scorifier is more liable to cause trouble than a shallow 
one, because the lead may completely cover the ore, while in a 
shallow one the ore will be semi-fused in the centre and surrounded 
by liquid lead. 

As to cause 6, ores such as AgCl, AgBr, and residues or pre¬ 
cipitates like those just mentioned as coming from the zinc boxes, 
seem most liable to spit in the scorifier, but the amount of spitting 
can certainly be diminished by observing every precaution pos¬ 
sible, especially by using broad and shallow scorifiers and keeping 
the muffle closed until the whole contents of the scorifier are thoroughly 
and completely fused and liquid. 

ASSAY OF ZINC RESIDUES FROM THE CYANIDE PROCESS. 

See Assay of Ores for Gold, page 163. 

ASSAY OF COPPER MATTE OR COPPER FOR SILVER. 

This can be made in one of three ways: 

1. By the ordinary scorification method. 

2. By special scorification method. 

3. By the combination wet and dry method. 

Method I. Take three portions of b A.T. of matte or of 
copper, place in a 3" or 3^" scorifier, mix with 35 grammes of 
granulated lead and place 35 grammes on top. Add £ to 1 gramme 
of very fine silica and 1 to ij grammes of borax glass. Scorify 
at as low a temperature as will not freeze or chill the assay. 


5° 


NOTES ON ASSAYING. 


When the lead eye covers, pour as usual and separate the 
lead from the slag. Weigh each button; add sufficient granulated 
lead to bring the total weight to 60 or 75 grammes and drop into 
three new scorifiers which are in the muffle and heated to a scorify¬ 
ing temperature. Add about 1 gramme of fine silica and \ gramme 
of borax glass to each, and scorify again at a low temperature. 

Repeat this second scorification until the color of the scorifier 
on the inner surface is light green on cooling. Cupel as usual. 
The color of the cupel should be greenish yellow and not black. 
The latter color indicates insufficient scorification. 

Method II. Into 3" or 3J" scorifiers weigh out three portions 
of A A.T. each. Mix with 35 to 50 grammes of granulated 
lead and spread 35 to 50 grammes on top. Add f to 1 gramme 
of very .fine silica and 1 to i| grammes of borax glass. Scorify 
at as low a temperature as possible that will not freeze or chill 
the assay. Allow the lead to slag over completely, remove the 
scorifiers from the muffle and pour off all the slag possible without 
pouring off any lead. Return to muffle and scorify until the 
lead button is judged to weigh between 8 and 12 grammes. Re¬ 
move the scorifier and pour contents, even if the lead has not 
slagged over. After a few trials the student will be able to judge 
the proper time to pour and have the buttons neither too large 
nor too small. The scorifiers will be black or dark green; if 
much iron is present, the brown color will obscure the green. 
Separate the buttons from the slag, and see that no lead is in the 
first slag poured off. 

Weigh each button, add sufficient granulated lead to each to 
bring the total weight to 75 or 90 grammes, and transfer to three 
new scorifiers which are in the muffle and heated to a scorifying 
temperature. Add 1 to ij grammes of fine Si 0 2 and J gramme 
of borax glass, and scorify as before. 

If, after this second scorification, the scorifiers are very light 
green , the buttons can be cupelled. If they are dark green, make 
a third scorification as before. If the material being assayed is 
of fair grade, the buttons can be cupelled separately, but if of 
low grade, all three buttons should be put in one cupel and three 
more assays should be started, if a check on the work is desired. 


ASSAY OF ORES FOR SILVER. 


5 1 


Weigh the buttons, part for gold, and deduct the amount found 
from the original weight of the button or buttons. 

Experiments carried out by Mr. H. T. Graber, class of 1903, 
upon a copper matte show the following interesting data in 
regard to the removal of the copper and the influence thereon of 
borax glass, silica, ordinary glass, and borax glass and silica 
together. 


FIRST SCORIFICATION. 


No. 

Weight 
of Matte 
Taken. 

Lead 
Taken 
(£ mixed 
with 
Matte, i 
on Top), 
Grms. 

Ratio of 
Pb to Cu 
in 

is A.T. 
of Matte. 

Ordi¬ 

nary- 

Glass. 

Silica 

(very 

fine), 

Grms. 

Weight 
of Lead 
Button. 

Copper in T V A.T. 
Matte = 1.584 
Grammes. 

Weight 

of 

Copper 
Removed 
in Slag. 

Per cent 
of Copper 
Removed 
in Slag. 

•n 

1 


To A.T. 

50 

31 to 1 

Pinch 

1 

15 

•7851 

49.6 

2 


( < 

< < 

< < 

( < 

< < 

13 

I.OOOO 

63.1 

3 


< C 

i ( 

< < 

< < 

< < 

12 

•6597 

41.6 

4 

A 

< i 

i < 

< < 

C i 

< < 

9 

.9910 

62.5 

5 

► a 

i < 

i < 

< < 

< ( 

< < 

9 

•8773 

55-4 

6 


< < 

< < 

< < 

( i 

< < 

15 

•5594 

35-3 

7 


i i 

C ( 

< < 

i < 

< < 

15 

•5963 

37-6 

8. 


i i 

( i 

< < 

( t 

< f 

9 

1.0163 

64.2 






Borax 










Glass. 





1 


* A.T. 

50 

3 1 to 1 

Pinch 

f 

10 

1.0114 

63.8 

2 


< < 

< ( 

< < 

< < 

< < 

11 

• 8549 

53-9 

3 


< < 

< < 

< < 

< t 

< < 

7-5 

.9626 

60.7 

4 


< < 

< < 

c < 

( C 

< < 

7-5 

1.0999 

69.4 

5 

T? 

i i 

< < 

< < 

C ( 

< < 

9 

1.0336 

65-3 

6 

£> 

( i 


< c 

< < 

< ( 

11 

.8808 

55-6 

7 


l ( 

( i 

i ( 

< < 

< < 

9 

.9182 

57-9 

8 


( < 

i i 

< < 

C ( 

< < 

9 

1.1150 

70.4 

9 


i i 

( ( 

C ( 

(( 

< C 

8 

1-2551 

79.2 

10 J 


i ( 

( ( 

< < 

< ( 

i C 

6 

1-2577 

79-4 

1 ' 


T V A.T. 

5 ° 

3 1 to 1 

3 

none 

10 

. 8864 

55-9 

2 


< < 

i ( 

( c 

3 

< < 

8 

•9534 

60.2 

3 

► u 

( i 

i l 

(( 

( < 

< < 

11 

• 6 505 

41.1 

4 J 


i i 

( i 

i c 

< < 

( C 

8 

.8072 

50.9 

5 


TV A.T. 

50 

31 to I 

none 

3 

6 

1.1384 

71.9 

6 


< i 

i C 

(i 

( c 

< c 

7 

•9574 

60.5 

7 

D 

i ( 

( i 

i ( 

i ( 

c c 

6 

1-0350 

6 5-3 

8J 


( c 

i i 

c < 

( ( 

c < 

7 

1.1165 

70.5 


* To make total lead 60 grammes at the beginning of second scorification. 
t To make total lead 35 grammes at the beginning of second scorification. 
f To make total lead up to 45 grammes in the third scorification. 















































52 


NOTES ON ASSAYING. 


The matte carried 54.3% copper, 

25 oz. gold, 

45 oz. silver. 

The work was done in a muffle fired with gas, and shallow 
3" scorifiers were used in all assays. 

In looking over this table it is evident that from 60% to 70% 
of the copper present is removed during the first scorification 




SECOND SCORIFICATION. 

THIRD SCORIFICATION. 







Per 







No. 

Silica 

Added, 

Grms. 

• 

<u 

<J 

rt 

i) 

Ratio of 
Pb to Cu 
Remain¬ 
ing in 
Button. 

Per 

Cent of 
Copper 
Re¬ 
moved. 

Cent 
of the 
Orig¬ 
inal 
Copper 
Pres- 

Silica 

Added. 

Lead 

Added. 

Ratio of 
Pb to Cu 
Remain¬ 
ing. 

Per 
Cent 
of Cu 
Re¬ 
moved. 

Per 

Cent of 
Orig¬ 
inal. 

Weight 
of Final 
Lead 
Button. 







ent. 







I 


1 

45 * 

75 to 1 

53-5 

26.9 


25 t 





2 


( i 

47 * 

126 to 1 

67.4 

24.8 

none 

140 to I 

94.O 

11 -3 

10 

3 


( < 

48* 

64 to I 

47.8 

27.9 


25 t 




4 

. A 

< < 

5 1 * 

IOI to I 

58.8 

22.0 

none 

100 to I 

45 • 1 

6.9 

4 

5 


< ( 

51 * 

84 to I 

49.9 

22.5 







6 


i ( 

45 * 

58 to I 

5 i -7 

33-5 







7 


i i 

20f 

60 to I 

46.3 

28.8 







8. 



i t 

26 f 

105 to I 

49.8 

17.8 







1 

2 

3 

' 


f 

24* 

24 to I 

42.3 

16.1 

none 

25 § 

24 to I 

15.2 

3-32 

8 

4 . 














51 


>B 












6 

7 

► 


f 

22* 

25 to I 

47-5 

17.9 

none 

25§ 

24 to I 

29.2 

5-78 

8 

8 J 














9 



f 

27t 

106 to I 

60.2 

12.5 







10 



4 

29t 

107 to I 

28.8 

5-9 


25 II 

150 to I 

13-9 

O T 

TO 


4 




1 ] 


Borax 





Neither 








Glass 





Borax 






2 

3 

4 J 

► c 

3 

23* 

20 to I 

31.0 

15.1 

Glass 

nor 

Silica. 

25 II 

17 to I 

10.1 

3-3 

10 

A 


Silica. 











O 

7 

■D 

3 

34* 

28 to I 

37-8 

12.5 

No 

Silica. 

25 I! 

27 to I 

4.0 

•83 


8 J 














Total lead was 33 grammes at the beginning of third scorification. 
To make the total 35 grammes. 

















































ASSAY OF ORES FOR SILVER. 


53 


and from 50% to 60% of the remainder during the second 
scorification. 

Silica evidently effects the slagging of the copper faster than 
borax glass, but a little of each in the first scorification seems to 
be most satisfactory; after that silica alone will do, although 
a little borax glass in addition does no harm. The ratio of the 
lead to the copper has a great influence on the amount of copper 
slagged, and the greater the ratio the more copper seems to slag. 
For this reason it seems advisable to use 70 or more grammes 
at first, and when four buttons from the first scorification are 
combined, to make the total lead up to 75 or 100 grammes rather 
than sixty. 

Metallic Copper, Copper Bars, etc.—Assay as in the case of 

mattes, Method II. A A.T. may be used, but it is generally 
better to take A A.T., unless the material carries very little 
silver. 

The following is an example of Method II. 


Copper Drillings. 


- 1 - A.T. 


TS 


Borax glass. 
Gran. lead.. 
SiO,. 


15 

1 gramme 
70 grammes 
1 gramme 
Scorified and poured slag off once. 
Placed again in muffle, scorified 
and poured. 

Lead button.. 6 6 grammes 


Scorifiers 3^"* 

A.T. 

1 gramme 
70 grammes 
1 gramme 


Lead added.68 


tV a.t. 

1 gramme 
100 grammes 
1 gramme 


A A.T. 

1 gramme 
100 grammes 
1 gramme 


Scorified and poured slag off twice. 


12 


8 grammes 


70 grammes 


Total lead .... 80 grammes 
Scorified as before with borax glass 
and Si 0 2 . 

Lead button.... 12 

Lead added. 58 grammes 

Total. 70 grammes . 

Scorified as before. 

Lead button-13 grammes. 

Cupelled with feather litharge crystals. 
Ag-f Au = . 04430 grammes 
= 22oz. 


90 grammes. 

Scorified as before with borax glass 
and Si 0 2 and poured slag off twice. 
9 grammes. 

Cupelled with feather litharge crys¬ 
tals. 

Ag+Au = 0.4454 grammes. 

= 222.7 oz. 

This cupel, owing to only two scori- 
fications, showed a little more coppei 
oxide than the other. 








NOTES ON ASSAYING. 


54 * 

In the scorification of copper ores or any cupriferous material 
I advise the continuation of the process until the scorifier is 
light green, for this color indicates that there is only a small 
amount of copper left in the lead and the cupels will be but 
slightly colored with the black oxide of copper. I know, however, 
that lead buttons full of copper are often cupelled, and if the 
silver beads blick, the assays are considered all right. This of 
course may save one scorification, but the cupels full of black 
oxide of copper are liable to carry much silver. 

Some assayers claim that, in assaying mattes and copper 
ores, the temperature should be extremely high when the scorifier 
is first placed in the muffle, and then dropped to a very low tem¬ 
perature, as soon as the lead commences to “ drive ” and kept so 
during scorification. 

Silver in matte, copper and copper bars is paid for on the 
basis of 95% of silver contents, and the price is that quoted on 
the day after the agreement of the assays. 

Combination Wet and Dry Methods. — There are several 
of these methods, and they generally give higher results for silver 
than the all-scorification methods. This no doubt partly accounts 
for the lack of uniformity of results by different assayers, and it 
seems only right, if umpire work is being done and assayers 
are checking each other, that the method used should be the 
same for one and all. 

The following method is one given by W. R. Van Liew in 
Eng. and Mining Jour ., April 21, 1900: 

“Take two or three portions of 1 A.T. each, place in beakers, 
add 200 c.c. of cold water and 100 c.c. of HNO a (sp. gr. 1.42). 
After a short period of action the beakers are placed on a steam- 
plate, and by the time the temperature has reached its maxi¬ 
mum (8o° C.) the copper is mostly dissolved. At the end of 
one hour complete solution has resulted, and at the end of 2J 
hours the beakers are removed, cooled, and 2 to 3 c.c. of normal 
salt solution, exceeding that amount necessary to precipitate all 
the silver present, are added, and the next morning the precipi¬ 
tate of AgCl is filtered into a double No. o 15-cm. Munktell’s 
Swedish filter-paper. Be sure and wash all the AgCl to the 


ASSAY OF ORES FOR SILVER. 


55 


extreme point of the filters. The wet papers are then placed 
in 2|-inch scorifiers containing, approximately, 6 grammes of test 
lead in their bottoms and burned to complete ash in a muffle 
not yet at incipient redness. The carbon burnt off, they are 
removed, when more test lead, litharge, and borax are added 
and the scorifiers replaced in the muffle. They are scorified 
at a low heat for approximately 20 minutes or until the lead but¬ 
tons weigh some 4 grammes. These resulting lead buttons are 
then cupelled at a temperature giving heavy litharge crystals. 
The time of operation is 24 hours.”* 

If gold is present in the copper bars or drillings, the method 
is conducted as per page 181, the resulting silver and gold bead 
is weighed, parted, and the amount of gold allowed for. 

Copper Mattes—Crucible Fusion. (See page 118.) 

CUPELLATION. 

The lead button from any scorification, which should be 
soft, malleable, and in the form of a cube with truncated edges 
and corners, weighing not over 30 grammes, is now ready for 
cupelling. This lead button should contain all the gold and 
silver in the ore and members of the platinum group, and our 
next step is to oxidize this lead to litharge, which is absorbed by 
the cupel, leaving the gold and silver and members of the platinum 
group, as a small bead, on the cupel. It is saje to warm the 
cupels on top of the jurnace and later on gradually push them 
into the muffle , heated to the full temperature. This gradual 
heating may prevent cracking. When heated red all through 
the lead buttons are carefully dropped into them, while they are 
in the furnace , the front cupels being charged first. If the cupels 


* References to assay of copper-material: 

A.I.M.E., vol. 24, p. 575. A. R. Ledoux. 

“ " 25, p. 250, 1000. “ 

“ “ 30, p. 529. L. D. Godshall. 

“ “ 30, p. 1121. 

Eng. & Min. Jour., vol. 65, p. 223. 

“ “ “ “ “ 69, p. 469. W. R. Van Liew. 

« “ “ “ “ 74, p. 650. T. B. Swift. 

Jour, of Analyt. Chem., vol. 6, p. 262. Prof. Whitehead. 




5 6 


NOTES ON ASSAYING. 


are thoroughly dry or warm, they can be placed directly in the 
hot muffle. 

Be sure that the cupel weighs more than the lead button and 
that the bowl will contain the lead without overflowing. 

The door of the muffle is now closed and the buttons fused 
as quickly as possible. When this is accomplished and the 
PbO begins to form, i.e., when the buttons have cleared or begin 
to “drive, ” the door of the muffle is opened and the temperature 
lowered. Make note of the time. The heat should be kept 
much lower than in scorification. Ij it is exactly right (625° 
to 775 0 C.), crystals oj PbO will be seen forming all around the 
inner surface of the cupel or on the front or cooler side , just above 
the button. 

If the heat is too high, no crystals of litharge will form, the 
whole cupel looks very hot, and the color of the litharge, absorbed 
by the cupels, is very indistinct. 

Above 775 0 C. silver will volatilize rapidly. (At 925 0 the 
loss is 3% to 4%, and at iooo° C. over 4%.) If the heat is too 
low, the cupels appear dark and cold; a film of litharge begins 
to form over the button of lead; it ceases “to drive”; then 
“freezes,” and the assay is rendered worthless. 

The lead button is now gradually oxidized by the air, most 
of the litharge, i.e., 90% to 95%, being absorbed as such by 
the cupel; the remainder going off as oxide (PbO), which is 
later on changed to carbonate of lead by the excess of air. 
As the button grows smaller it will be noticed that it becomes 
more round and that the beads or drops of litharge, which are 
continually thrown from the centre of the button towards the 
circumference become larger and appear like drops of oil upon 
water. This indicates that the button is near “blicking” or 
“brightening,” and the heat should be slightly raised or the cupel 
pushed back into the hotter part of the furnace. As the last of 
the litharge goes off the button, one will notice a brilliant play of 
colors and the button seems to be agitated and revolving upon an 
axis. As the last of the colors disappear, the button becomes dull 
and after a second or so looks bright and silvery. Again make 
note of the time and see how much lead has oxidized per minute 


ASSAY OF ORES FOR SILVER. 


57 


during cupellation. This last part of the cupelling process is 
called the “brightening” or “blicking.” 

The cupel should be immediately drawn out from the muffle 
far enough to have the button solidify. When the button solidi¬ 
fies it will again flash or glow. Good buttons should separate 
easily from the cupel, have a silvery lustre or surface, be round 
if small and hemispherical if large, dullish white upon the bot¬ 
tom, and have no rootlets. Large buttons should be withdrawn 
very slowly or else covered over with a hot cupel; otherwise they 
will “sprout ” or “vegetate .” 

Silver buttons containing much gold seldom, if ever, sprout. 

Sprouting is said to be due to the button suddenly giving 
off the oxygen which it has absorbed during the cupelling process. 

Sprouted buttons as well as those which have jrozen should be 
rejected , as they generally give low results even if brought again 
to “ driving ” by the addition of fresh lead or charcoal or both. 
A cupel, provided the bowl is large enough , will absorb about its 
own weight of litharge, and the different oxides absorbed color it 
as follows: 

Lead colors it yellow; copper , black to greenish black; 
iron , black and leaves a black scoria on the sides. Zinc causes 
boiling and will, if present in large amount, crack the cupel. 
Antimony cracks the cupel to pieces if present in large amount; 
if present only in small amount, it will simply cause a roughening 
of the edges of the cupel. Rings of light-colored scoria may be 
due to the oxides of arsenic, antimony, zinc, or tin. 

Nickel , if present above J%, will leave a blackish-green scum 
of nickel protoxide. The button will “drive” at first, but will 
finally leave the scum all over the cupel. The lead goes into 
the cupel as PbO as usual. Under \% of nickel the lead button 
will cupel, but a green coating is generally left upon the cupel. 

The heat in cupelling should always be lower than in scorifying , 
but should , especially in the case of large beads or of gold buttons, 
be raised at the period of “blicking” in order to keep the silver 
or gold melted while the last traces of lead and litharge in the 
button are being removed. 

When the bead is perfectly cold it is seized by the button- 


53 


NOTES ON ASSAYING. 


pincers, detached from the cupel, to which it should adhere only 
slightly, and brushed with a stiff brush. If this 
does not clean it thoroughly, it should be placed 
upon its side on a small anvil, hammered, and 
then brushed again. The button , which must be perjectly cold r 
is now ready to be weighed. 

The balance upon which this is done should be sensitive 
to too of a milligramme. Such balances should be handled with 
the utmost care and should be in perfect adjustment before any 
weighing is attempted. All buttons should be weighed to the 
fifth place of decimals (thus: .05063 grammes), and should be 
reported in ounces and fractions thereof (thus: 60.3 oz.). 

EXAMPLE. 


No. 1. No. 2. 


Weight of ore taken. 


iV A.T. 

Lead used. 

. 35 

i( 

45 grammes 

Weight of Agd~ Au obtained. .. , 


U 

.00640 “ 

Silver contained in the Pb used. 


ii 

.00037 “ 

Weight of Ag-|-Au in ore.. 

.00603 

<( 

.00603 “ 

Ounces per ton. 



60.3 


Value at market rate, say 50c. =$30.15 per ton of 2000 lbs. 

“ Control ” or “ check ” assays are generally done in triplicate* 
If two results agree, the third is discarded; if all three disagree, the 
ore is again assayed. “ Umpire ” work often necessitates six assays. 

The chief loss in the silver assay occurs during cupellation. 

The lead buttons should really be free from impurities of all 
kinds, but in reality they generally contain some. For example, 
suppose we take the lead buttons from the ores previously scorified 
(see page 43); some of the reactions which probably take place 
in cupelling are as follows: 

The button is lead carrying Au, Ag, and a very little Sb, Zn, 
and Cu as impurities. We admit air into the muffle when 
we are cupelling this button, and we have Pb+0 = PbO, which 
is partly absorbed by the cupel and partly volatilized; as this 
PbO comes in contact with the air, it is converted into lead 
carbonate and coats the furnace white outside the muffle. 

The Sb 2 0 3 is partly volatilized and partly carried into the 
cupel along with the PbO: 











ASSAY OF ORES FOR SILVER. 


59 


2Sb+3O = Sb 2 0 3 ; 2Sb+3PbO = Sb 2 0 3 + 3Pb. 

TheZnO is partly carried into the cupel and partly volatilized: 

Zn + 0 = ZnO; Zn+PbO = ZnO + Pb. 


If much zinc is present, it will burn and oxidize, giving off a 
very brilliant greenish-white flame. 

The CuO is absorbed by the cupel and colors it black: 


Cu +0 = CuO; Cu+PbO = CuO + Pb. 


The Ag and Au are not oxidized, but are left on the cupel with 
the exception of a very small amount, which is partly carried into 
the cupel by the PbO and partly volatilized with the PbO. These 
losses are dependent largely upon the heat used, the texture of 
the cupel itself, and the size of the lead button. 

The following will give some idea of what becomes of the 
lead during the cupellation process: 


No feather crystals 
of PbO upon 
cupels. 


Weight of 
Button of Lead. 


PbO Absorbed by Cupel. 


Percentage of Pb 
Button Absorbed 
by Cupel. 


■ 30.2 

grammes 

30-9 

grammes 

= 28.67 Pb 

94.9 

28.6 

< < 

29.2 

i i 

= 27.10 

< t 

94.8 

28.4 

c ( 

28.8 

C ( 

= 26.73 

( c 

94.1 

28.2 

< c 

28.7 

i i 

= 26.64 

( i 

94.4 

16.0 

c c 

16.4 

( i 

= 15.22 

t i 

95 -i 

15-3 

< < 

15-7 

(i 

= 14-57 

C ( 

95-2 

14.9 

t ( 

15.2 

(< 

= 14.18 

C ( 

95-2 

. 13-3 

i c 

13-5 

(( 

= 12.52 

< ( 

94.2 

36.0 

< c 

37-5 

i c 

= 34.80 

< < 

96.0 


In the last the cupel weighed 31 grammes when new; after 
use it was completely full of PbO and weighed 68J grammes. 

The principal loss in the silver assay occurs in cupelling; part 
of this loss is due to volatilization, but the chief source of error 
is occasioned by absorption of the silver by the cupel. This is 
easily shown by taking a known weight of silver-foil (C.P.), 
wrapping it up in some C.P. lead-foil, and carefully cupelling it. 

Experiment with C.P. Silver.—Weigh out accurately upon 
the button-balance one portion of C.P. silver-foil, from .2 to .4 
of a gramme, say .29918 grammes. 






t)0 


NOTES ON ASSAYING. 


Weigh out between 6 and 7 grammes of C.P. lead-foil upon 
the pulp-balances. 

Wrap the C.P. silver in the C.P. lead. 

Weigh a cupel from which the bone-ash does not rub off 
easily and heat to cupelling temperature. 

Drop the button of lead and silver into the cupel. 

Cupel with feather litharge crystals. Note the time from the 
driving to the blicking of the button. Have a hot cupel at hand 
to cover the one in use when the button blicks. Remove, clean, 
and then weigh silver button and weigh cupel again. 

Now take the cupel, cut off all the bone-ash not colored with 
PbO, and grind the remainder through a 60-mesh sieve. Weigh. 
Assay for silver by the crucible method, using 50 PbO, 20 borax, 
5 silica, and 2\ argols, or 60 PbO, 10 borax glass, 5 silica, 15 soda, 
and 2\ r-~ols (R.P. = 10). These charges will do when the. 
weight 0 / bone-ash plus PbO, minus the actual weight oj litharge , 
does not exceed 15 to 20 grammes; above this the litharge, 
silica, and especially the borax glass will have to be increased. 
Cupel the resulting lead button. Weigh the silver bead and 
deduct the silver in the PbO used, if any is present. 


Report as follows: 

Weight of C.P. silver taken, for instance.29918 grammes 

“ “ “ “ after cupellation. 2 959° “ 


Silver lost during cupellation.00328 = 1.09% 

Weight of silver found in cupel, less correction 

for Ag in PbO used.00288 

Percentage of silver absorbed by the cupel.96 

“ “ “ lost by volatilization.13 

Weight of C.P. lead taken. 6.00 grammes 

“ cupel+PbO. 27.95 “ 

“ cupel before using. 21.81 “ 


u 


u 


Weight of PbO. 6.14 “ 

Weight of lead calculated from the PbO = 5.69 grammes. 

From the PbO absorbed, calculate the per cent of the original 
lead that is in the cupel = 94.83%. 














ASSAY OF ORES FOR SILVER. 


6r 


If the silver button recovered from the cupel, less any silver 
in the litharge used (.00288 grammes), is more than is indicated 
by the loss in cupelling (.00328), that is, if in the foregoing test 
the weight .00288 grammes had come out .00342 grammes then 
there must have been lead either in .29590 grammes, making 
the loss (.00328) too small, or else there was lead in .00342 
grammes. 

Such a result may occur if the cupel is not pushed back into- 
the hotter part of the furnace just before the button blicks. 

It is seen from this experiment that the resulting silver button 
weighs considerably less than the amount of silver originally 
taken, and that about 90 per cent of this loss is due to absorp¬ 
tion by the cupel. 

This loss is influenced by: 

1. The cupel, whether hard or soft. A hard cupel may not 
absorb the PbO as fast as made, thus prolonging the operation 
and increasing the loss of silver. A cupel may be so soft that, 
the PbO will carry silver into it. 

2. The character and fineness of the bone-ash of which the 
cupel was made. 

3. The amount of silver cupelled. The larger the amount 
of silver cupelled the greater is the loss of silver, but the smaller 
is the percentage loss. 

4. The amount of lead used. Beyond a certain size of but¬ 
ton the loss of silver will increase because the time of cupellation 
will be unnecessarily prolonged. 

5. The presence of base metals in the button. If a button 
contains much copper, CuO will be formed with the PbO and 
this, when absorbed by the cupel, seems to drag silver with it 
into the cupel. The presence of Sb will increase the loss of 
silver both by absorption and volatilization. 

6. The temperature at which the cupelling is carried on.. 
Feather crystals of litharge should appear on the inner surface 
of the cupel, usually in front, otherwise the temperature is too 
high. 

7. The quantity of air passing into or through the muffle. 


62 


NOTES ON ASSAYING. 


Too little air delays the operation, causing loss; too much air 
may make the operation too rapid. 

The following tables are taken from the thesis of Messrs. F. 
J. Eager and W. W. Welch, class of 1902, who investigated these 
Josses. They used a gas-muffle, made their own cupels, and a 
•sample of the bone-ash they used sized as follows: 


On 20-mesh sieve 


Through 

20 on 

40. 

1 c 

. _ (( 
40 

60. 

(( 

60 “ 

80. 

< < 

80 “ 

100 

< < 

100 “ 

120 

(( 

120... 



2.10% 
1.00 
3.00 
5.20 
13.00 
9.40 
68.20 


This bone-ash required only 10% of water to make it of the 
xight consistency before pressing it in the cupel-machine. 


SILVER LOSSES IN CUPELLATION. 

The following table shows the importance of cupelling at 
the correct temperature. 

Lead and silver constant, temperature varying. 


C.P. Silver 

Lead. 

Time, 

Temp. 

Per- 

Remarks. 

used in 

Minutes. 

c. 

centage 

Grammes. 


Loss. 


.20462 
.20606 

10 grammes 

<< i ( 

15 to 18 

U 

700° 

<< 

•99 

1.05 

) Crystals of PbO 
all about the 
) button. 

.20427 

t i a 

15 

775 
< < 

1.18 

| Crystals of PbO 

.20010 


16 

.76 

on cooler side 

.20472 

a a 

15 $ 

< < 

1.41 

) of cupel. 

*20554 

a a 

i6| 

00 

0 

1.70 

No more crystals. 

.20030 

< i n 

15 

< < 

1.81 

< < 

.20140 

i i n 

15 

C < 

1.69 

< ( 

.20518 

a a 

Not taken 

850 

i -75 

< < 

.20016 

a a 

<< << 

to 

1.69 

< < 

.20300 

«« < < 

a a 

870 

1.80 

< < 

.20172 

i ( (< 

11 

9 2 5 

2-59 

i i 

.20380 

i i a 

10 

< < 

3-53 

< < 

.20347 

a t < 

11 

( ( 

3-78 

< C 

.20129 

< i a 

14} 

1000 

4.78 

< < 

.20586 

a t ( 

15 

< < 

4-97 

< c 





























ASSAY OF ORES FOR SILVER. 63 

The temperatures were taken by a Le Chatelier pyrometer, 
and as the junction could not be held in the lead button, it was 
kept about above it. 

It was found that the temperature there was about ioo° 
higher than above the floor of the muffle at the same spot, due 
no doubt to the oxidation of the lead itself. This explains why 
buttons can be cupelled at a comparatively low temperature 
after they have started to drive. In order to give a quick drive 
the buttons were melted at 775°; the temperature could then be 
lowered to 625° and the buttons kept driving, but towards the 
Wick” the temperature had to be raised to 750° or 775 0 . To 
cupel at as low a temperature as 625° can be done even in 
practice, but it requires care and attention. At this temper¬ 
ature crystals of litharge form all about the button. The range for 
these crystals seems to be from 625° C. to about 8oo° or 825°. 
The temperature of the muffle varied about 400° from the front 
to the back. Looking at the table, it is seen that the silver loss 
increases rapidly as soon as no more crystals are obtained on the 
cupel, and it is for this reason that the cupelling temperature 
should always be sufficiently low to obtain these crystals. The 
cupels should, however, always be pushed back into the hotter 
part of the furnace just before the button blicks, otherwise it 
will be apt to carry lead. 

Another thing noticed in this run was, that as the temperature 
increased the tendency of the buttons to sprout increased. At 
1075 0 it was almost impossible to keep them from sprouting. 
Furthermore, as the loss increased, the color of the cupels, where 
soaked with litharge, became more green, which was evidently 
due to the increased amount of silver absorbed. 

The following table shows the effect of varying the amount 
of lead. 

Silver and temperature constant. 

Three determinations were made at one time, and all condi¬ 
tions were kept as nearly alike as possible. 


64 


NOTES ON ASSAYING . 


No. 

Silver. 

Lead. 

Temp. 

C. 

Percentage 

Loss. 

Mean of the 
Two Nearest 
Together. 


f 1 

.20517 

IO 

grammes 

68 5 ° 


f i -35 


« 

2 

.20168 

< < 

< < 

< < 

i 

1.70 

i *39 


l 3 

.20404 

< < 

< < 

< < 


[ 1.42 



4 

• 2 °555 

15 

( i 

( i 


r i. 3 ° 


* 

5 

.20651 

< < 

( ( 

tt 

i 

1-45 

1.38 


6 

.20077 

( < 

C < 

t i 


1.14 



f 7 

.20182 

20 

< < 

<< 


i .49 

\ 

4 

8 

.20284 

< < 

< i 

«« 

• 


1.85 

1.52 


L 9 

.20318 

( i 

C i 

<« 


l 1-55 



r 10 

.20225 

25 

€ i 

t i 


' 1.82 


4 

11 

.20517 

< < 

i i 

1 i 

i 

1.87 

1.85 


12 

.20632 

< c 

C < 

< ( 


1.22 



The table below shows the effect of varying the amount of 
copper. 

Silver, lead, and temperature constant. 


No. 

Silver. 

Lead, 

Grammes. 

Temp. 

c. 

Copper. 
Per Cent of 
the Silver. 

Percentage 

Loss. 

Mean of the 
Two Nearest 
Together. 

1 

.20382 

IO 

775 ° 

5 


C 1. 00 


2 

.20256 

6 6 

6 < 

6 i 


I . IO 

I.05 

3 

.20036 

C 6 

< < 

t i 


l *90 


4 

.20618 

11 

<« 

10 


f 1.19 


5 

.20193 

< c 

i C 

< i 


1.09 

I.08 

6 

.20118 

( < 

( c 

< c 


1.06 


7 

.20146 

< < 

( c 

15 


f 1.27 


8 

.20138 

< i 

< < 

< < 


1.30 

I.29 

9 

.20432 

( < 

c c 

( i 


l 1 - 35 


10 

.20282 

( < 

c t 

20 


f I J 5 


11 

.20100 

c < 

tt 

i < 


i .45 

i -45 

12 

.20338 

i c 

i c 

( c 


l 1.46 


13 

.20224 

( < 

< c 

25 


1.05 

) Copper in 

14 

.20496 

C i 

< t 

< < 

4 

•95 

j- the silver 

i 5 

.20420 

c < 

< c 

(( 


l. !- 0 7 

) buttons. 


Many things worthy of note were observed in this series. 
The presence of copper made it necessary to have the tempera¬ 
ture of the muffle 900° in order to make the lead drive. The 
temperature was then lowered to 775 0 . The presence of 5% and 
10% of copper seemed to have very little influence on the loss of 
silver, but it is to be remembered that the ratio of lead to copper 
is over 400 to 1. 

Above 10% copper the silver loss increased, and when 25% 















































ASSAY OF ORES FOR SILVER . 


6 5 


was reached the silver buttons retained copper notwithstanding 
the ratio of lead to copper was 200 to 1. As the per cent of 
copper increased the tendency of the silver buttons to sprout 
increased, which no doubt is due to copper being such a good 
absorbent of oxygen. 

The Effect of Tellurium on the Loss of Silver.—A series of 
tests with the silver constant, lead constant (10 grammes), and 
temperature constant at 775 0 , but the tellurium varying from 
2.2% to 15%, showed as follows: 

The loss of silver seemed to remain about normal, and the 
buttons appeared bright and clear until 15% of tellurium was 
used, when the resulting silver beads had a dull and frosted appear¬ 
ance on the surface. These buttons, when dissolved in strong 
H 2 S 0 4 , gave the characteristic pink color, showing the presence 
of tellurium. 

The student can easily see from the foregoing tables how 
careful he should be in the cupelling operation, and how he should 
endeavor always to have feather litharge crystals upon the cupel. 

The silver losses taking place in both scorification and cru¬ 
cible work may be summed up as follows: 

1st. Silver carried into the slag, which is the smallest loss. 

2d. Silver volatilized during cupellation, which is next in 
amount. 

3d. Silver absorbed by the cupel, which occasions the largest 
loss. 

In the ordinary work of an assay laboratory it is impossible to 
assay all the slags and cupels so as to determine these losses, but 
they can be determined, and in the assay of very rich material and 
in special cases the corrections are made as in the Assay of 
Zinc Residues, page 163. 

Parties sending samples to different assayers should specify 
whether they wish this to be done or not, for I have known samples 
to vary simply because one assayer was making the corrections 
and another was not. 

Silver Beads of Unusual Appearance.—Beads containing 
certain ratios of silver and gold, when cupelled at too low a tem¬ 
perature near the blicking point , instead of continuing to drive> 


66 


NOTES ON ASSAYING. 


flatten out, giving a gray, mossy bead. This is due to the presence 
of some 8% to 12% of lead. Such beads, if wrapped in lead foil 
and recupelled at a higher temperature, will blick and give bright, 
rounded buttons. 

Buttons of this character should not be mistaken for those 
containing a large amount oj platinum , which flatten out and 
have an appearance somewhat similar. Repeated cupellations 
will not alter the appearance of these buttons. 

When platinum is present in small amount the silver bead 
is rough, irregular, and a high temperature is required to blick it. 
Tellurium causes a silver or gold bead to appear dull and frosted. 

ASSAY OF ORES FOR SILVER, CRUCIBLE METHOD. 

(POT-FURNACE; COKE FUEL.) 

The object here, as in the scorification method, is to form a 
slag from the gangue or waste material of the ore, and to collect 
the precious metals by means of lead, which is cupelled afterwards. 

In the scorification method our chief fluxes are lead and borax 
glass, with an addition of silica in some cases and sometimes a 
pinch of soda. 

The free access of air gives an oxidizing atmosphere. In 
the crucible process, on the other hand, although we make use of 
oxidizing agents, such as litharge (PbO) and nitre (KNO a ), we 
are generally using reducing agents, and the crucible being covered 
up, the atmosphere is more of a reducing one. 

FLUXES AND REAGENTS. 

The principal fluxes and decomposing agents used are the 
following: 

Sodium Carbonate (melts at 814° C.) and Potassium Carbon¬ 
ate (melts at 885° Le Chatelier). Both are most important 
fluxes. Either bicarbonate of soda or sal-soda may be used, 
because cooking-soda is decomposed by heat as follows: 

2NaHC0 3 Theat = Na2C0 3 -t- CO 2 TH2O. 

Both the carbonates act as basic fluxes and combine with silica 
and silicates to form a silicate of the alkali with the disengagement 
of C 0 2 : 2 Na 2 C 0 3 + Si02 = 2Na 2 0,Si02 + 2C02. 

This gas tends to make them oxidizing agents. 


ASSAY OF ORES FOR SILVER. 


67 


The silicate of soda formed may be one of the following, 
depending upon the ratio of the silica to the carbonate of soda 
in the fusion: 

4Na20,Si02 = subsilicate. 

2Na 2 0,Si02 = monosilicate. 

Na20,Si02 = bisilicate, i.e., 62 to 60 or about neutral. 

2Na 2 0,3Si02 = trisilicate. 

4Na 2 0,3Si02 = sesquisilicate. 

The following charges, fused in a pot-furnace in an E crucible, 
will illustrate this: 


Bicarbonate of soda, grammes 

No. 1. 

.... 24 

No. 2. 

24 

No. 3. 
22 

No. 4- 
20 

Si 0 2 , 

• 

• 

• 

• 

W 

O 

l6 

20 

25 


No. 1 poured clean and well, and the resulting product was 
probably the sesquisilicate of soda. Slag very stringy. 

No. 2 poured out, but was a little thick. 

No. 3 poured part way out of the crucible. 

No. 4 only partly fused. 

Both carbonates form fusible compounds with many metallic 
oxides, but the compounds are not stable and are readily broken 
up by the presence of carbon. Carbonate of soda I consider 
one of the most important if not the most important flux used 
in assaying sulphide ores by the crucible method. It decomposes 
these sulphides and in the case galena with a reduction of lead. 

7PbS + 4K 2 C03 = 4Pb + 3 (K 2 S,PbS) + K2SO4 + 4CO2. 

If Na2C03 + 2C are heated in a closed vessel, we have 
2Na + 3CO, or 2Na 2 C03 + C = 2Na 2 0-f-2C02. 

Also Na 2 0 -f C = 2Na-f CO. 

Then PbS + Na 2 0 + C = Pb+Na 2 S + CO .* 

The amount of lead thrown down seems to depend upon 
the amount of alkali used, as shown in the following fusions. 


* See also page 97. 




68 


NOTES ON ASSAYING. 


Galena (carrying 84# lead), grammes 15 / 15 IS 


Bicarb, of soda, “ 10 4 ° 75 

Borax, “ 10 — — 

Glass, “ 5 5 5 

Cover of salt in each case. 

Time of fusion, minutes. 25 25 25 

Lead, grammes. None 9-15 9.89 or 

of the lead 
present 

Lead matte, grammes. A little None None 

Slag.Thick, did not 

pour well — — 

Slag, color. Black Black Black and gray 


The carbonates may be used indifferently, but Na2C03 is to be 
preferred, as it is cheaper and does not deliquesce. 

Both carbonates together make a more fusible mixture than 
either one alone. The bicarbonate NaHCOs is generally used, 
because it is more likely to be free from sulphates. 

Borax, or biborate of soda (Na 2 0 , 2B2O3, or Na2B 4 0 7 -K 
ioH 2 0 ), melts at about 560° C. Soluble in water. 

This is an excellent and universal flux. It is neither oxidiz¬ 
ing nor desulphurizing, but forms fusible compounds with all 
the bases, and fuses and combines with most of the metallic oxides. 
15 to 20 grammes will make 6 grammes of MgO perfectly liquid, 
while 60 grammes of PbO will not. Owing to the presence of 
boracic acid it acts as an acid flux, but is not as strong a one as 
Si 0 2 . Too much in a fusion has the same effect as too much 
Si 0 2 , rendering the fusion thick. Its influence on the size of 
the lead button may be to increase or diminish it, depending 
upon the amount used and the character of the ore. It may 
be used in the form of biborate of soda, but owing to the large 
percentage of water in this (47.2%), which causes much swelling 
in the crucible or scorifier, it is better in all scorijication work 
and crucible jusions in the muffle to use borax glass. 

Borax Glass. —This is ordinary borax fused (loss about 40 
per cent), poured into moulds, and later on broken up into small 
pieces. It is almost twice as strong as common borax, and costs 
25 cents or more per pound. On account of this high cost it 
should be used only for muffle-work and for refining bullion. 

Litharge (sp. gr. 9.2 to 9.36; Pb = 92.86%, 0 = 7.14%).— 








ASSAY OF ORES FOR SILVER. 


69 


Melts at about 950° C. Quick cooling is said to promote the 
yellow color, slow cooling the red color. It is a strong oxidizing 
-agent, oxidizing all the metals except Ag and Au; also all the 
sulphides, arsenio-sulphides, etc.: 

Fe + PbO = FeO + Pb; FeS 2 + 5PbO = FeO + 2 S 0 2 + 5PI); 

Zn+PbO = ZnO+Pb; ZnS + 3 PbO =ZnO +S 0 2 + 3 Pb. 

It is a universal flux, forming fusible compounds with bases 
and combining with Si 0 2 to form lead silicates. 

The silicate formed depends upon the ratio of silica and 
litharge present in the fusion, and this fact should be borne in 
mind when silica is added to a charge. 

The silicate formed may be one of the following: 

4Pb0,Si0 2 = subsilicate. 

2PbO,Si0 2 = monosilicate and most readily fusible. 

2 PbO, 2 Si 0 2 = bisilicate. 

2 Pb 0 , 3 Si 0 2 = trisilicate. 

These are all fusible, but above this they commence to become 
infusible, and when we have 2 Pb 0 ,i 8 Si 0 2 the mass will become 
only pasty even at a very high temperature. 

Metallic iron will decompose these silicates either partly or 
wholly, with a reduction of lead: 

2 Pb 0 ,Si 0 2 + 2 Fe= 2 Fe 0 ,Si 0 2 + 2 Pb. 


The following fusions made in an E crucible in the pot-fur¬ 
nace will show the effect of Si 0 2 in a fusion when both soda 
bicarbonate and litharge are present. 



No. 1 . 

No. 2 . 

No. 3. 

No. 4 . 

No. s. 

No. 6 . 

Sodium bicarbonate, grammes. 

IS 

15 

15 

15 

15 

15 

Litharge, “ . 

60 

60 

60 

60 

60 

60 

Argols, “ . 

3 

3 

3 

3 

3 

3 

Si0 2 , “ . 

24 

26 

3° 

33 

3 6 

39 

Cover of salt in each case. 







Ratio of Si0 2 to soda. 

1.6 to 1 

1.7 to I 

2 to I 

2.2 to 1 

2.4 to 1 

2. 6 to I 

Time of fusion, minutes. 

35 

35 

25 

25 

25 

25 

Lead, grammes. 

27 

26 

28 

24 

24 



No. 1. Slag poured well, and was glassy. No. 2. Slag poured well, was glassy and green in 
color. No. 3. Slag was thick and contained some lead. No- 4. Slag was much thicker than No. 3 
and contained more lead. No. 5. Slag was lumpy. No. 6. Slag would not pour. 


























70 


NOTES ON ASSAYING. 


Although litharge is a strong base, it forms fusible com¬ 
pounds with oxides infusible by themselves. Fusible mixtures 
are thus formed with lime and the earths, which, though bases 
themselves, seem to be held in solution. In the crucible assay, 
although acting as a flux, its principal use is to supply the lead 
to alloy with and collect the silver and gold in the ore. When 
brought into contact with carbon , organic matter , metallic sul¬ 
phides or iron , it is reduced to metallic lead , and this, while settling 
in a spray through the contents of the crucible, collects all the 
silver and gold in the ore: 

2 PbO+C = 2 Pb+C 0 2 ;* 

9 PbO + Sb 2 S 3 = 9Pb + Sb 2 0 3 + 3 S 0 2 ; 

2 PbO + PbS = 3 Pb+S 0 2 ; 

PbO + Fe =FeO+Pb. 

Iron.—This is a desulphurizing agent and separates the sul¬ 
phur from Pb, Ag, Hg, Bi, Zn, Sb, Sn, and partly from Cu: 

PbS+Fe = FeS + Pb. 

It also decomposes litharge thus: PbO + Fe = FeO + Pb; owing to 
which reaction some assayers use it to throw down the lead 
:n their fusions, but I wish the student to consider it as a 
°-ulphurizer. 

A decomposes lead silicate as follows: 

2 PbO,Si0 2 + 2Fe = 2Fe0,Si0 2 + 2Pb. 

In the crucible assay it is used either in the form of nails 
and spikes, which are put in point down, or as iron wire, which 
can be twisted into any desired form. 

Iron and Arseniate of Soda.—From the following fusions it 
is seen that arseniate of soda when present in a fusion with litharge 
and soda does not reduce lead nor form a speiss with iron when 
the latter is added. 


* When an oxide is easily reducible, as PbO, the gas given off will be CO . 
When not easily reducible, as ZnO then CO is formed. 




ASSAY OF ORES FOR SILVER. 71 


Arseniate of soda, grammes: 
Bicarb, of soda, ‘ ‘ 

5 

0.6* 

5 

0.6* 

IQ.2 

30 

30 

3 ° 

3 ° 

3 ° 

Borax, ‘ ‘ 

— 

— 


5 

5 

Litharge, ‘ ‘ 

30 

30 

30 

30 

3 ° 

Silica, ‘ ‘ 

— 

— 


3 


Iron nails (20-penny). 

Cover of salt in each case. 

3 

3 

3 

Time of fusion, minutes. . .. 

3 ° 

30 

30 

3 ° 

30 

Lead, grammes. 

* 25 t 

• 3 I t 

27.62 

23-65 

16.4 

Speiss, “ . 

None 

None 

None 

None 

None 

Slag, color. 

White 

Drab 

Black 

Black 

Black and 
thick; lead 
globules 
present 

Salt, “ . 

.Yellow with 
red spots 

Yellow with 
red spots 

— Grayish 


* Corresponding to 3 /,„ A.T. of an ore containing 2 o 5 /, 0 % of arsenic, 
f The small amount of lead thrown down in the first two fusions is no doubt 
due to the presence of some arsenite of soda in the arseniate. 


Iron and Arsenite of Soda. 

— The following fusions show that 

in the presence of litharge an arsenite like a 

sulphide 

throws 

down lead and that the presence of 

iron does 

not necessarily 

form a speiss. 





Arsenite of soda, grammes. 

2.7 

5-4 

2.7* 

5-4 

Bicarb, of soda, “ . 

30 

3 ° 

30 

30 

Borax, “ . 

— 

— 

5 

— 

Litharge, “ . 

3 ° 

3 ° 

30 

30 

Silica (Si 0 2 ), “ . 

— 

— 

3 

3 

Iron nails (20-penny). 

— 

— 

3 

3 

Cover of salt 

in each case. 



Time of fusion, minutes. 

3 ° 

30 

30 

30 

Lead, grammes. 

4.8 

8 

26.9 

27.8 

Speiss, “ . 

None 

None 

0.42 

None 

Slag, color. 

Gray 

Gray 

Black 

• 

Black 

Salt, “ . 


Yellow and red 



* Corresponding to 3 / 10 A.T. of an ore containing 2 o 5 / ]0 % of arsenic. 


Charcoal; Argols, KHC 6 H 4 0 6 ; Cream of tartar, C 4 H 5 K 0 6 ; 
Sugar, Ci2H22O11; Starch; Flour (Reducing Power (R.P.) about 
15).—These are all reducing agents, i.e., they are capable of re¬ 
moving oxygen from those compounds with which it may be com¬ 
bined. They are used in the crucible assay to remove oxygen from 
the PbO, and to reduce the necessary amount of lead to collect all 
the precious metals in the ore. They have different reducing 




















72 


NOTES ON ASSAYING. 


powers, and assayers prefer some one and some another. Char¬ 
coal is itself infusible and does not combine with fluxes; too 
much will therefore render an assay thick and infusible. Flour 
is always easily obtained, so it is most commonly used. I prefer 
crude argols or cream of tartar, because on heating they break 
up into carburetted hydrogen, carbon monoxide, K 2 C 0 3 , KHO, 
and finely divided carbon, and for this reason act both as a flux 
and a reducing agent. One objection raised against their use 
is that they cause the fusions to boil excessively. 

Potassium Nitrate (melts at 339 0 C.) and Sodium Nitrate 
(melts at 316° C.) (both neutral to litmus).—These are both 
powerful oxidizing agents. They fuse without alteration at a 
temperature below redness, but when heated more strongly they 
give up oxygen: 

2PbS+ 2KN0 3 = 2 Pb+ K 2 S 0 4 + S 0 2 + 2N; 

2Cu 2 S+ 2KN0 3 = 4Cu+K 2 S0 4 + S 0 2 T 2N. 


(If nitre is used in excess, the slag will contain Cu 2 0 and PbO.) 

In the above way the nitrates readily decompose the sul¬ 
phides, arsenides, etc., in the ore; the oxygen set free readily com¬ 
bines with the sulphur, forming S 0 2 and the sulphate of the alkali 
used. 

They do not oxidize metallic lead very rapidly unless it is 
very finely divided and suspended in a molten mass, as shown 
in the following tests: 


Granulated lead, grammes 
Nitre (KN 0 3 ), “ 


No. 1 . No. 2 . No. 3 . 

, .... 60 | 60 (bottom of crucible) 60 

• * • • J 5 J crucible. *5 ( on t0 P of lead) 15 

Salt cover salt cover salt cover 


Resulting lead button.42 

Fusion, minutes.20 


44 

42 

20 

20 


For the determination of the oxidizing power (O.P.), seepage 
81. 

Fe 2 0 3 and Mn 0 2 .—These are both oxidizing agents and 
are basic in action: 


Fe 2 0 3 + C = 2FeO + CO. 








ASSAY OF ORES FOR SILVER. 


73 


Bear this reaction in mind when assaying an ore containing 
either of these, especially in the case of roasted concentrates 
which previously contained iron pyrites. They may have such a 
strong oxidizing power that no lead button will be found from 
a fusion where 3 grammes of argols (R.P. 10) are used. 



No. 1. 

No. 2. 

No. 3. 

No. 4. 

Fe„ 0 3 , grammes. 

20 

20 

20 

i A.T. of roasted concen- 

Litharge, “ . 

60 

60 

75 

trates 

40 Na 2 C 0 3 

Argols, “ . 

2 

2 

7 

20 borax 

(R.P. = 9 ) 

Glass, “ . 


IO 

TO 

60 litharge 

4 argols (R.P.= io.2) 

IO 

Cover of salt in each case. 

The resulting lead button 





weighed, grammes. 

9 


55 

8 

If the Fe 2 O s had not been present 
the lead button should have 



weighed (grammes). 

18 

18 

63 

C 

CO 

6 



Oxidizing power of ore = or i . i 


Silica (Si 0 2 ).—This is a strong acid flux. It is used when 
the bases in an ore are in excess, when the ore is deficient in 
gangue matter, and also to protect the scorifiers and crucibles 
from the action of litharge. (See under Litharge, page 69, for 
the silicates formed.) For the effect of too much Si 0 2 in a 
fusion, see pages 69 and 96. 

Glass.—This is ordinary window-glass or chemical glass¬ 
ware ground fine. It is already a silicate of the alkalies, lime, 
lead, or all of these, so its influence upon a fusion is not the same 
or as marked as silica. The ingredients of the glass are already 
wholly or partly in combination, while the silica is free. (See 
page 84.) Its use in crucible work is recommended for those com¬ 
mencing assaying, for it seems to act as an equalizer in the charge 
and is especially advantageous in the fusion of black sands from 
sluice-boxes and similar material. Such products generally 
contain a variety of minerals mostly acting as bases and it is 
usually necessary to add both borax and silica. One can easily 
add too much silica and have trouble in the fusion, whereas 
a little too much glass will do no harm. 




















74 


NOTES ON ASSAYING. 


Fluorspar—A most excellent flux for baryta or heavy spar. 

Salt—This is used as a cover to the charge to keep out the 
air and to clean the interior surface of the crucible, preventing 
the small particles of lead from adhering thereto. It smelts at 
772 0 C. (Le Chatelier). 

Some assayers object to the use of salt, claiming that it is 
of no advantage and in some cases causes the crucibles to crack. 
This has not been my experience, however. 


CRUCIBLE EXPERIMENTS WHICH MAY CLEAR UP SOME OF 

THE FOREGOING. 


1. 14 grammes PbO+3i grammes Fe gave 12 grammes of 
Pb and a slag, glassy and red in color. 

2. 14 grammes PbO + 3.5 grammes Fe-b.3 grammes Char¬ 
coal. Pb = i3 grammes, i.e., all the Pb. Slag black and dull. 

3. 15 grammes PbO + 6.2 grammes Tap Cinder (FeO) + f 
grammes Charcoal. Pb = 4 grammes. Slag black and infusible, 
lead scattered all through it. (15 grammes PbO carrying 92.8% 
Pb = i3.92 grammes Pb.) 

4. 42 grammes of lead placed in the bottom of a crucible, 
covered with slag, fused 20 minutes, and poured gave 42 grammes 
of lead. 


5. 15 grammes PbS + 3J grammes Fe gave lead = 12^ 
grammes (PbS-f Fe = FeS+Pb), also an iron matte. 

6. 15 grammes PbS-f 6.2 grammes Tap Cinder+f grammes 
Charcoal. Pb = 9 grammes (Fe+PbS = FeS+Pb) and an iron 
matte. The slag was infusible, probably due to too much C. 
(FeO+C = CO + Fe.) 

7. 15 grammes PbS+9 grammes Fe gave Pb=i3 grammes; 
also iron matte. 

It is claimed that when an excess of iron is not used some 
Pb will go into the iron matte. No. 5 seems to confirm this, 
although the amount of Pb is very little less than No. 7. For 
if 56 parts iron will reduce 207 Pb (Fe+PbO = FeO+Pb), one 
part iron will reduce 3.69 Pb. 

In 15 grammes PbO there are 13.92 grammes Pb. 

. I3-92 


3-69 


= 3.77 grammes Fe to reduce all the Pb from the PbO. 



ASSAY OF ORES FOR SILVER. 


75 


The effect of fluxes at a high temperature on different sub¬ 
stances. 


The amount of each flux used is the same as would be taken 
in an ordinary assay. 

MgO. 

No. of fusion. i 2 3 4 

MgO.6 grammes taken in each fusion on the basis 

that 1 A.T. of ore might contain 20 %. 

Bicarb, of soda, grammes. 30 — — — 

Borax, “ — 20 15 — 

Litharge, “ — — — 60 

Silica, “ — — 5 — 

No. 1. Was infusible. 


No. 2. Was the most liquid; slag was glassy. 

No. 3. Was next to No. 2 in fusibility, slag was glassy. 

No. 4. Was a little lumpy; slag was dull in appearance. 
Magnesia being a base, borax and silica are the best fluxes, 
for they act as acids. Litharge in large excess can be used; 
its action seems to be that of dissolving the MgO within itself. 


Clay. 

No. of fusion. 

Clay. 

Bicarb of soda, grammes 
Borax, 

Litharge, 


567 

12 grammes taken in each fusion on the basis 
that 1 A.T. of ore might carry 40%. 

3 ° 

— 20 

— — 60 


No. 5. Was very thick and would just pour; slag pasty. 
No. 6. Was liquid, but very stringy; slag was glassy. 

No. 7. Fused, but was very lumpy. 

Here again borax seems to be the best flux. 

Fe 2 0 3 . 

(A Lake Superior hematite carrying 93.57% Fe 2 0 3 and a total of 4.88% of 
Si 0 2 and Al 2 O a in about equal amounts.) 


No. of fusion. 

.10 11 12 13 

14 

15 

16 

17 

Fe 2 0 3 (iron ore). . 

.12 grammes were taken on the 

basis that 1 A.T. of ore might 
carry 40%. 

15 

15 

1 A.T. 

Bicarb, of soda, grammes 24 36 — — 

40 

40 

30 

60 

Borax, 

“ — — 20 — 

20 

— 

20 

20 

Litharge, 

“ — — — 60 

60 

60 

60 

60 

Silica, 

a _ _ _ _ 

— 

4 

7 

12 

Argols, 

“ —. — — — 

4 

4 

4 

4 















7 6 


NOTES ON ASSAYING. 


No. io. Was a long time fusing and would only just pour 
after a very high temperature of an hour. 

No. n. Was the same as No. io, only slightly more 
liquid. 

No. 12. Was fused in about 20 minutes and poured rather 
thick at the end of half an hour. 

No. 13. Was fused in about 15 minutes, except a slight 
scum on top. Poured well, except a slight scum, at the end 
of half an hour. The slag from this and No. 12 was slightly 
magnetic. 

No. 14. Gave a good fusion after 35 minutes. Slag 

glassy. 

No. 15. Poured afer 35 minutes’ fusion, but was rather thick. 
Slag was dull in appearance and seemed basic. 

No. 16. Poured well after 35 minutes’ fusion. 

No. 17. Poured well after 35 minutes’ fusion. Crucible only 
slightly attacked. 

The lead buttons from Nos. 14, 15, 16, and 17 weighed 
between 30 and 35 grammes. 

Ferric oxide being a strong base, fusions Nos. 15, 16, and 17 
show that fluxes acting as acids, like borax and silica, are absolutely 
essential. Considerable borax should be used, and it seems safe, 
in order to form a silicate of iron, to add sufficient Si 0 2 so that 
the amount added plus what is judged to be in the ore shall 
be 30% to 40% of the ore used. Rathei high PbO seems advis¬ 
able, and the temperature at which the fusion is conducted should 
be very high. 

TV3O4. 

No. of fusion. 16 17 18 19 20 21* 

Fe 3 0,.. grammes were taken on 15 15 

the basis that 1 A.T. of 
ore might carry 40%. 

Bicarb, of soda, grammes. 24 36 — — 30 30 

Borax, “ . — — 20 — 15 _ 

Litharge, “ .. — — — 50 50 90 

Argols, “ . — — — 3 3 

Silica, “ . — — — — ^ ^ 










ASSAY OF ORES FOR SILVER . 


77 


Nos. 16 and 17 would just pour after 25 minutes’ fusion; 
slag magnetic. 

No. 18. Poured, but was thick; slag magnetic. 

No. 19. Poured at the end of 10 minutes; fusion very liquid, 
crucible nearly eaten through. Slag magnetic. 

No. 20. Very liquid after 30 minutes’ fusion, but slag carried 
some lead. Crucible eaten into a good deal. In this fusion 
there would be no free litharge remaining if each gramme of 
argols reduced 9 grammes of lead and each gramme of silica 
combined with 7 grammes of litharge. 

No. 21. A good liquid fusion yielding a 30-gramme lead 
button. Slag was very clean, crucible not much attacked. There 
are about 25 grammes PbO free in this fusion. 

The conclusions to be drawn from these fusions seem to be 
as follows: 

To ensure a good liquid fusion and a slag free from lead, 
fluxes acting as acids, like borax and silica, must be used to com¬ 
bine with the iron oxide, which is a base. The litharge must 
be high and in excess and some soda, as usual, is necessary. 
The temperature at which fusion is conducted must be very 
high. 

Glass may be used in place of silica. 

Mica ( Muscovite ). 


No. of fusion. 8 9 10 

Mica.6 grammes taken in each fusion on 

the basis that 1 A.T. of ore 
might carry 20%. 


Bicarb, of soda, grammes. 30 — — 

Borax, “ — 15 — 

Litharge, “ — — 60 

No. 8. Was very thick and would not pour from the cru¬ 
cible. 

No. 9. Was liquid but was thicker than No. 10; slag glassy. 
No. 10. Was very liquid, slag very glassy. 

Litharge is evidently the best flux here. 







78 


NOTES ON ASSAYING. 


Sulphates.—Action in presence of litharge. 

Zinc Sulphate .—This acts neither as an oxidizing nor reducing 
agent. 

ZnS 0 4 , grammes. 

Bicarb, of soda, “ . 

Litharge, “ . 

Glass, “ . 

Argols (R.P. 9.6) “ . 

Cover of s 

Time of fusion, minutes. 

Lead, grammes. None 

Slag, color. Spotted 

Salt, “ . 


reducing agent. 


45 


grammes. 


CaSO 
Bicarb, of soda, 
Litharge, 

Glass, 

Argols (R.P. 9.6) 


Time, minutes. 

Lead, grammes. None 

Slag, color.Full of spots, Clear 

crust in cru¬ 
cible 


10 

10 

10 

None 

10 

10 

80 

80 

80 

5 

5 

5 

None 

None 

2 

in each case. 


25 

25 

25 

None 

None 

19.25 

Spotted 

Spotted 

Yellow 

— 

— 

Yellow 

1 neither 

as an 

oxidizing 

10 

10 

10 

None 

10 

10 

80 

80 

80 

5 

5 

5 

None 

None 

2 

in each case. 


25 

25 

25 

None 

None 

18.6 


Yellow 


Lead Sulphate .—No reduction of lead takes place. 


PbSO 


4 > 


grammes. 


10 

10 

80 

5 


Bicarb, of soda, “ . 

Litharge, “ . 

Glass, “ . 

Cover of salt. 

Time, minutes. 25 

Lead. None 

Slag, color.Yellow gran¬ 

ular, crust 
in crucible 


Sulphates.—Action in presence of litharge and sulphide of 
lead. 

Calcium Sulphate acts neither as an oxidizing nor reducing 
agent in presence of either or both. 






















I 


ASSAY OF ORES FOR SILVER . 


79 


Calcium sulphate, grammes. 

Bicarb of soda, 

Borax, 

Litharge ( 92 8 /io% Pb) 

Galena (84% Pb), 

Glass, 

Cover of salt in each case. 

Time, minutes. 

Lead and lead matte, grammes.. . 

Lead, “ 

Slag, color. 

* According to the reaction PbS+ 2PbO = S0 2 +3Pb, the button of lead should 
weigh 38.5 grammes. 


10 

10 

10 

10 

10 

10 

None 

80 

15 

15 

5 

case. 

5 

2 5 

2 5 

10.6 

— 

— 

37 - 94 : 

— 

Clear and 

2 +3Pb, the button < 


Lead Sulphate .—When fused with lead sulphide a reduction 
of lead takes place, the amount brought down depending upon 


the amount of alkali used. 

Fusion No. 

1 

2 

3 

4 

5 

6 

Lead sulphate (if pure, 68.3% 
Pb), grammes. 

10 

10 

10 

10 

10 

10 

Bicarb, of soda, “ . 

40 

20 

10 

20 

20 

60 

Galena (84% Pb), “ . 

None 

None 

15 

15 

15 

15 

Glass, “ . 

5 

5 

5 

5 

5 

5 

Argols, “ . 

None 

2 

None 

None 

2 

None 

Cover of salt in each case. 

Matte, grammes. None None Large 

-5 

2.9 

None 

Lead, “ . 

-44 

6.8 

Small 

14.6f 

10. of 

16.45 

Slag, color. 

* 

Brown 

bead 

— 

.. 

Black 


* Thick, last of it very thick; when cold, yellow all through and bluish yellow 
on top. 

t Lead was brittle, caused, no doubt, by presence of sulphur, which was also 
present in the slag in considerable amount. 

Fusion No. i shows that a slight amount of lead is thrown 
down by the bicarb, of soda and No. 2 shows that all the lead 
is reduced in the presence of a reducing agent and sodium bicar¬ 
bonate. 

TESTING REAGENTS. 


One of the first things an assayer must do is to test the purity 
of his reagents. Lead and litharge can both be obtained free 
from silver and gold, but this purity is only brought about by 
special refining. As gold is more readily removed than silver 
the former is less likely to be present than the latter. Some 
lead and litharge on sale carry considerable silver and sometimes 
gold; therefore we find it absolutely necessary to assay every new 
lot received, and as some lots run very unevenly, they require 
just as careful sampling as any ore. 



















8 o 


NOTES ON ASSAYING. 


Granulated Lead.—If this cannot be readily purchased, it 
can be made by melting lead at as low a temperature as possible, 
pouring it into a box and shaking it slowly, in a horizontal direc¬ 
tion, until it begins to congeal or become pasty; it is now shaken 
very rapidly until it granulates. Sift through a 12-mesh sieve 
and remelt what does not pass through. The loss by this method 
will not be over one per cent. It can also be made by blowing 
steam through a stream of melted lead. 

Testing jor Silver and Gold. —Assay for silver and gold by 
scorifying three or four portions of 120 to 160 grammes each in 3" 
or 4" scorifiers. If necessary, rescorify the resulting buttons and 
continue to do this until the buttons are sm.dl enough to cupel. If 
the lead runs very low in silver and gold, two or more buttons 
may be combined in the scorifier or cupel. Weigh the bead of 
precious metals and part for gold. Make the corrections to 
apply to 35, 45, and 50 grammes of lead. This correction has 
to be made even if extremely small, for otherwise silver and 
gold might be reported as being present in an ore when it was 
entirely absent. 

Litharge .—Testing jor Silver and Gold. 


Pot-furnace. F, G, or H Crucible. 



' 4 A.T. PbO 

Mix 

20 grammes soda 

* n . 

10 “ borax glass 

cru¬ 

cible. 

3! ‘ ‘ argols * 


4 ‘ ‘ silica 


V. 

Cover of salt. 


Muffle-furnace. A or B Crucible. 


60 grammes to 3 A.T. PbO 
10 “ soda 

8 “ borax glass 

3 J “ argols * 

2 ‘ ‘ silica 

Cover of salt. 


Mix 

in 

cru¬ 

cible. 


* Reducing power = 8. 


The 28 grammes of lead thrown down by the argols will 
collect all the silver and gold in the whole amount oj PbO used . 
Weigh the bead and part to see if gold is present. 

In assaying some samples of lead and litharge it is necessary 
to take large amounts of each, because they carry very small 
amounts of the precious metals; therefore if we take 35 grammes 
of lead or 30 grammes of litharge, we may not obtain a bead and 
yet silver or gold may be present. For this reason, especially when 
assaying ores which have a very small amount of silver in them, 
unless C.P. reagents are used, I prefer lead and litharge carry- 






ASSAY OF ORES FOR SILVER. 


Si 


ing so much silver that a bead will result when 35 grammes 
are used, rather than lead and litharge which has a correction 
of, say, .00011 grammes for 35 grammes, figured from the assay 
of 120 or 160 grammes. A correction as small as .00011, to 
be accurate, can only be obtained by using a large amount of 
lead or litharge. If now we make an assay of an ore carrying 
a very small amount of silver and use only 35 grammes of lead 
or litharge, the silver bead both from the ore and the lead or 
litharge will very likely weigh less than .00011 grammes. 

Oxidizing Power (O.P.) of Nitre.—Nitre melts at about 339 0 
C. Like litharge it is a strong oxidizing agent and has the prop¬ 
erty of oxidizing sulphides with ihe formation of S 0 2 and sul¬ 
phate of potassium. 

The oxidizing power should be found by fusing it with an ore 
the working reducing power of which is known. 

The following are examples: 

FeS2 and 



Arsenopyrite. 

Concentrates. 

Chalcopyrite. 

Ore, grammes... 

• 3 

3 

3 

3 

2 

3 

Sodium bicarb. “ . .. 

. 6 

3 

6 

6 

2 

3 

Litharge, “ . .. 

• 5 ° 

60 

5 ° 

5 ° 

90 

90 

Nitre, “ . .. 

. — 

4 

— 

4 

— 

4 

Silica, “ . .. 

. — 

3 

— 

— 

I 

3 


Cover of 

salt. 

Cover of salt. 

Cover of salt. 

Time of fusion, minutes. . 

. . 17 

19 

20 

15 

20 

20 

Lead, grammes. 

. 21.50 

4.71 

25.76 

8-34 

18 

9.2 

R.P. 

. 7.17 


8.58 


9 



21.50 







4.71 







4)16.79 






O.P. 




4-35 


4-45 


The average value orf this lot of nitre, after many fusions with 
different ores, was found to be 4.3, and this value was confirmed 
by the size of the lead buttons, when the regular assays of the 
ores were made and a large amount of nitre used. 

It is often claimed that the oxidizing power of nitre varies 
with different ores , but the variation is no more than shown in 
the previous fusions, provided the right reducing power oj the ore 
is used . If the true reducing power was known and every condi¬ 
tion kept the same in each fusion, the oxidizing power would 
probably be found to vary not as much as indicated. 









8 2 


NOTES ON ASSAYING. 


The following will illustrate how easily a wrong value for 
nitre may be obtained: 


No. of Fusion.. 155 
Ore, grammes... 3 
NaHC 0 3 , “ ... o 

Litharge, “ ... 60 

Nitre, “ ... — 

Time, minutes. 13 

Temp., deg. C. 1240 

Lead, grammes. 14*72 

R.P. 4.91 


14 s 

146 

147 

3 

3 

3 

3 

6 

9 

60 

60 

60 

Salt 

cover in 

each case, 

10 

10 

10 

1320 

1330 

1290 

18.93 

23.26 

23 * 3 2 

6.31 

7-75 

7-77 


117 

118 

156 

2 

3 

3 

2 

6 

0 

80 

70 

60 

— 

— 

4 

19 

19 

13 

1225 

1120 

1265 

14.03 

23.02 

4*95 

7.02 

7.67 



The R.P. of this ore is 7.7. 

If the oxidizing power of nitre is based on fusion 155, it equals 

^ or 2.44. If on fusions 146 and 147 it is 4.5. If on 
4 

fusion 117 it is 4.02. 

Either of the values 4.5 or 4 is close enough for practical work. 

To figure the O.P. from the regular ore fusions is not safe, 
unless these charges are made up on exactly the same basis as the 
preliminary fusion and conducted in the same way, jor the size 
oj the resulting lead button depends upon the amount 0 / soda, 
borax , litharge, and Si 0 2 added to the charge, the amount oj gangue 
in the ore, and the temperature at which the jusions are conducted. 

The oxidizing power does seem to vary, based on the lead but¬ 
tons, with different reducing substances like argols, charcoal, 
flour, etc., as shown in the following fusions. When the nitre is 
kept constant and the litharge varies, as in fusions Nos. 55 and 47, 
the oxidizing power varies, which seems to indicate that the varia¬ 
tion is due to the fluxes, temperature, or something other than 
the nitre. Therefore do not use the oxidizing value found in this 
manner, when making up a charge for an ore. 


Argols, grammes... 

3 3 

3 Charcoal 1 

No. 55. 

1 Starch 2\ 2$ 

No. 47 
2$ 

Soda, “ 

3 3 

3 

3 

3 

3 

3 

3 

Litharge, “ 

60 60 100 

60 

60 

60 

100 

60 

Nitre, 

— 4 

4 

—• 

4 

— 

4 

4 

Si 0 2 , “ 

3 3 

3 

3 

3 

3 

3 

3 


Cover of salt. 

Cover of salt. 

Cover of salt. 

Temp., deg. C.... 

1060 1225 

1090 

1160 

1210 

1150 

1160 

1220 

Lead, grammes.... 

29.78* 12.47 

12*55 

27-57 

8.42 

30 - 38 t 

10.54 

9.64 

O.P. of nitre. 4.33 4.30 

* Average of four fusions. 

4 • 8 

t Average of two fusions. 

4.96 

5.18 









ASSAY OF ORES FOR SILVER. 


S3 


Reducing Agents (Charcoal and Argols).— Testing jor Reduc¬ 
ing Power ( R.P. ).—Before the student attempts to assay an ore 
by the crucible method he should determine the reducing power * 
of his reducing agents, as charcoal, argols, and flour. The object 
of this is twofold: 


1 st. To obtain their values in order to know what amount of 


them to use in the regular fusion of the ores. 

2d. To learn the principal steps connected with a fusion in a 
crucible. Take two crucibles, either E or F. 

Into them , in the order given , weigh out carefully the following: 


<D 


<U 


S £ 

O •( 

-a 2 

S u 


grammes 
<.1 

( c 


' Litharge. 60 

Bicarb, soda.. 3 

Argols. 3 

Silica (Si0 2 ). . 2 

or 

l Glass. 5-10 

Weigh out the argols and the charcoal 
flux-balance. 

Cover of salt, deep. 


Litharge.... 60 

grammes ' 


Bicarb, soda. 3 

C C 

0 

Xi ai 

-4-> 

G £ 

L 0 

Charcoal. . . 1 

< i 

Silica. 2 

i i 

.2 2 

or 

a 


Glass. 5-10 

i i 

• 



on the pulp-balance, the others on the 
Cover of salt, deep. 


The mixing in the crucible is done by holding the crucible 
slightly inclined, and while revolving it in one hand, with the 
iron spatula continually bring the material up from the bottom 
of the crucible. When finished, hit the crucible sharply all 
round to settle the contents and remove any material clinging 
to the inside above the charge and then put on the cover of salt. 

The cover of salt, when melted, keeps out the air, washes 
down and cleans the sides of the crucible and makes a glaze, thus 
preventing the lead globules from sticking to the sides of the cru¬ 
cible. Have a good bright fire, then sprinkle over it a thin layer 
of fresh coke, to prevent the crucibles coming directly in con¬ 
tact with the hot coals, and next place the crucibles in the furnace; 
put a cover on each crucible to keep out all dust and coke; care¬ 
fully pack coke around them, and do not disturb in any way 
until the contents fuse. The top of the crucibles should be below 
or only slightly above the bottom of the flue. Urge the fire and 

* When we speak of the reducing power here, we mean the amount of lead 
that 1 gramme of the substance will reduce or throw down from an excess of 

litharge. 













8 4 


NOTES ON ASSAYING. 


heat the crucibles until the contents begin to fuse, then check the 
fire and see that the contents oj the crucibles do not boil over. When 
the contents are fairly quiet, put the draft upon the fire and heat 
at a high temperature (noo C. or over) for 15 to 20 minutes 
or until the fusion is perfectly quiet. The reaction which has 
been going on is 2PbO + C = 2Pb + C 0 2 , which will continue until 
all the carbon present has been oxidized by the oxygen in the PbO 
present. Take the crucible out of the fire, and pour the fusion 
into a mould which has been coated with chalk , ruddle (Fe 2 0 3 ), or 
oil y previously heated and dried. When cold, separate the lead 
from the slag, hammer into a cube, and weigh to the first place oj 
decimals on the pulp-balance. Suppose they weigh 29 and 24 
grammes respectively; it means that the reducing power of 1 
gramme of charcoal is 29 grammes of lead, and of argols 8 grammes 
of lead ( ? 3 4 ), and that they will reduce this amount of lead from 
an excess of PbO, whether the amount of PbO is 60 or 1000 
grammes. The excess of litharge remains as litharge or com¬ 
bines with a portion of the crucible and forms a lead silicate. 
If we use only 20 grammes of litharge, some of our reducing 
substance would be left unoxidized and our reducing power 
would be too low and therefore inaccurate. Both fusions can 
be made without the silica (Si 0 2 ) or the glass, which are added to 
form a slag with the excess of litharge present and to prevent the 
PbO from combining with the constituents of the crucible and 
eating through. The glass has less effect than silica, because it is 
already a silicate of lime, soda, potash, lead, or a mixture of 
these, and is preferable for those commencing assaying. The 
silica is Si 0 2 and has a great influence upon the results, as the 
following fusions, made in a pot-furnace, will show. 


Bicarb, soda, gms.. 







3 

6 

12 

3 

Litharge. 

60 

60 

60 

60 

60 

60 

60 

60 

60 

60 

Argols. 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

Glass 

, 

LQ 

20 

__ 

_ 










9 

C 

10 





Cover of salt in 




0 

O 




3 


each case. 












' Temp, outside of 












crucible, deg. C. 

1220 

1940 

1250 

940 

940 

940 

1280 

*345 

I 3 2 ° 

1060 

, 

Lead button, gm. 

29.03 

28.9 

27.7 

28.7 

28.03 

24.81 

29.69 

30-83 

30.75 

29.28 


. Reducing power. 

9.68 

9- 6 3 

9.24 

9-55 

9-34 

8.27 

9.90 

10.28 

10.25 

9.76 


Time of fusion between 12 and 20 minutes. 
























ASSAY OF ORES FOR SILVER. 


85 


In other words, when 60 grammes of PbO and 3 grammes of 
argols are used, the addition of soda, up to 6 grammes, increases 
the size of the lead button; beyond this it has little if any effect. 
The limit for the silica is evidently between 2 and 3 grammes, 
for 3 grammes diminish the size of the lead button, and 5 grammes 
have a marked effect. If the lead silicate 2 PbO,Si 0 2 is formed 
(a ratio of 7 PbO to 1 Si 0 2 ), then no excess of PbO would be left 
in the fusion when 5 grammes of Si 0 2 were used, and carbon 
cannot reduce lead from a lead silicate. 

The Si 0 2 used contained 98.68% Si 0 2 . The glass seems to 
have about one fifth the effect of the silica, i.e., 1 Si 0 2 = 5 glass. 

The results, when using charcoal, were as follows: 


Bicarbonate of soda, grammes. 

3 

_ 

_ 

Litharge, “ . 

60 

60 

60 

Charcoal, “ . 

1 

1 

1 

Glass, “ . 

— 

— 

10 

Silica, “ . 

Cover of salt in each case. 

3 

3 

" 


f Time, minutes. 

15 

15 

15 


Temperature, degrees C. 

1225 

1300 

ii 57 


Lead, grammes. 

27.85 

27.38 

27-57 


Reducing power. 

27.85 

27.38 

27-57 


The effect of too much silica or glass will be shown when the 
value of any other reducing agent is obtained in the same way 
by substituting it in place of argols or charcoal and keeping the 
rest of the charge the same. 

The reasons for not making these fusions with litharge alone 
and the reducing agent are twofold: first, because by using 
the silica and soda we approximate somewhat to the charge for 
the regular fusion; and second, because a crust is prevented. 
A fusion may be at a very high temperature and liquid below, 
but a crust on top will prevent a clean pour. Small amounts of 
silica, soda, or borax prevent this, and the whole charge will be 
liquid. If, when these fluxes are present, a crust forms or the 
fusion is thick, it is due to either too low a temperature or an 
excess of some flux. 

The effect of borax and borax glass is similar to that of silica, 
although not so marked: 

















86 


NOTES ON ASSAYING. 


Litharge, grammes. 

60 

60 

60 

60 

60 

60 

60 

60 

Argols, “ . 

3 

3 

3 

3 

3 

3 

3 

3 

Borax-glass, “ . 

— 

3 

5 

10 

— 

— 

— 

— 

Borax, “ . 

— 

Cover 

of sal 

t in 

3 

each 

6 

case. 

9 

12 

Time of fusion, minutes. 

12 

13 

19 

16 

10 

11 

10 

10 

Temp, outside crucible, deg. C.. 

1220 

860 

860 

860 

1240 

860 

1225 

1330 

Lead, grammes. 

29.03 

28.39 

28.49 

27.22 

28.77 

2 7 • 5 1 

26.78 

25-99* 

Reducing power. 

9.68 

9.46 

9-5° 

9.07 

i 

9-59 

9.17 

8.92 

8.66 


From this table 6 grammes of borax have about the same 
effect as io grammes of borax glass. 

Incorrect results may also be obtained by not having the heat 
high enough, as shown in the following: 

Pot-furnace. B Crucible. Muffle. B Crucible. 


0 

Bicarb, soda. ... 

3 

gm. 

3 

3 

3 

3 

• r* J 

Litharge. 

60 

< < 

60 

60 

6a 

CJ 1 

5 

Argols. 

3 

< < 

Charcoal i 

Argols 3 

Charcoal i 

O 

[ Glass. 

10 

i i 

Glass 10 

Glass 10 

Glass 10 


Cover of salt 



Cover of salt 

Cover of salt 

Cover of salt 


Lead. 

3 1 • 1 

i i 

27-5 

K> 

00 

H 

26.4 


Reducing power 

10.36 


* 27-5 

9.4 

26.4 


ASSAY OF ORES FOR SILVER: CRUCIBLE METHOD. 

Having tested the reducing agents and found their values, to 
be used in all subsequent work, we can now take up the assay of 
an ore. The principal advantage of the “crucible method,” 
whether for assaying ores for silver or for gold, is that we can use 
large amounts of ore. It is therefore especially adapted to 

(a) Poor or low-grade ores, i.e., ores poor in silver and gold. 

(b) Refractory ores, or those with a refractory gangue like 
limestone, barite, etc., which require large amounts of borax 
glass or some other flux to decompose them in the scorifier. 

(c) Special ores like chloride of silver, which spit badly in 
the scorifier. 

Avoid ij possible using the method jor ores containing large 
amounts oj copper , antimony , and like metals , which are liable to 
be reduced and pass into the lead button, and hence to necessitate 
a scorification. (See special methods, pp. 118 and 122.) 

We may divide the ores for crucible work into: 
































ASSAY OF ORES FOR SILVER. 


87 


Class I. Silicious, oxide, and carbonate ores or ores con¬ 
taining no sulphides, arsenides, etc., i.e., ores with no reducing 
power, or which are unable to decompose litharge with a reduc¬ 
tion of lead. 

Class II. Ores carrying sulphides, arsenides, or organic 
matter, i.e., ores having a reducing power or ores which can 
decompose litharge with a reduction of lead. 

The character of the sample of ore can of course be most 
readily determined when the ore is in a coarse condition or in 
lumps, but as fully half the samples received by the assayer are 
in a pulverized condition, he must be able to form a very close 
idea of their composition. 

Given a sample of pulverized ore to assay either for silver or 
gold, or for both, the student should ask himself the following 
questions: 

(a) Is the ore sufficiently fine to assay, i.e., will it pass through 
a 100-mesh sieve or a finer one? 

(b) What is the character of the sample, i.e., what minerals 
are present? Are they sulphides, oxides, carbonates, or other 
compounds ? 

(l c ) Is the sample apparently an iron, copper, lead, or zinc 
ore, or is it a mixture of several minerals ? 

(l d ) Is there much gangue and is this gangue acid or basic, 
i.e., is the gangue quartz and silicious, or does it consist of basic 
material, such as iron oxide or limestone? 

( e ) Is the sample better adapted for the scorification or for 
the crucible method ? 

All these questions have a bearing upon the actual assay; 
for, as in chemistry we use certain methods in the separation of 
certain elements, so in assaying certain methods must be used 
upon certain classes of ore. 

The fluxes, reagents, sizes of scorifier, crucibles, and heat 
used depend upon the nature and composition of the sample. 

Given an ore, let the student empty the whole of it out of its 
receptacle and mix or roll it 100 times on oilcloth or glazed 
paper. Now take a very small portion of it, place it in a horn 
spoon, a dish, vanning-shovel, or gold-pan, moisten it with water 
and shake it gently; this will cause any heavy material that may 


88 


NOTES ON ASSAYING. 


be present to settle out. By gently washing off the lighter portion 
the student can examine the heavy portion or concentrates, if any 
are present, and decide whether it belongs to Class I or Class II. 

Having decided whether the ore contains sulphides or not, 
proceed to weigh out the fluxes on a flux balance and place them 
in the crucible. The ore is weighed out last oj all on a pulp-balance , 
and brushed from the scale-pan into the crucible, where it should 
be thoroughly mixed with the fluxes. 

If the ore is weighed out first it is apt to be left at the bottom, 
where it will merely sinter , stick to the crucible , and never be 
decomposed. 

Some prefer to mix the ore and fluxes on paper and then 
transfer the mixture to the crucible, but this seems to me unneces¬ 
sary, for a thorough mixing can be done in the crucible, thereby 
avoiding losses on paper and in transferring. 

The amount of ore taken can be any weight from ^ A.T. 
to 4 A.T., but the fluxes must be in proportion. The crucible 
should never be more than two thirds full when the charge is all 
in and the cover oj salt placed on top. 

Class I. Ores under this class are assayed upon the following 
plan. (Pot-furnace, E or F crucible.) 


O . 
a; 

o 

2 

S 


Silicious Ore. 


Charge (a). Charge ( b ). 

’Ore. \ A.T. Ore ..JA.T.' 

Bicarb, soda, grammes.. 30 Bicarb, soda, grammes.. 15 

• Borax, “ 0-3 Borax, “ 0-5 

Litharge,* “ 30 Litharge,* “ 60 

. A rgols,t “ 3 l Argols,t “ 3I . 

Cover of salt £ inch thick over all. 

* Sufficient to supply 25 to 28 grammes of lead, 
f Each gramme reduces 8 grammes of lead. 


« • 
0 



Each fusion should give a lead button weighing 26 grammes. 

The fluxes are always weighed out first and placed in the cruci¬ 
ble, and the ore last oj all. Mix thoroughly in the crucible , strike the 
crucible several times on the outside , and then place the cover of 
salt on top, which washes down the interior of the crucible and 
prevents excessive boiling of the contents. 

For the reducing power oj argols, take the value you find when 
testing reagents , page 83. Other reducing agents, like charcoal, 
flour, starch, etc., can be used, but their reducing power must 
be known. 







ASSAY OF ORES FOR SILVER. 


89 


As ores are often assayed in duplicate, the student is recom¬ 
mended to vary the charge used and therefore to make one 
fusion as per charge (a) and the other as per charge ( b ). A 
good plan to follow is to have the amount of active fluxes two 
or more times greater than that of the ore used. For instance, 
in charge (a) the soda is high and the litharge is low. The 
latter disappears to give the lead button, and the argols disappear, 
with the exception of a little KOH and K 2 C 0 3 , which we may 
neglect. Therefore the active fluxes remaining are 30 grammes 
of soda and 5 grammes of borax, or 35 grammes in all, i.e., a 
little more than twice the ore used (J A.T.). If we put the 
soda down to 15 grammes, then we should have only 20 grammes 
of active fluxes, and with many ores this would be insufficient to 
insure a good liquid fusion. 

In charge ( b ) the soda is low and the litharge is high, so our 
active fluxes are 15 grammes of soda, 5 grammes of borax, and 
30 grammes PbO, or 50 in all, which is 3J the amount of ore, and 
the fusion will probably be more liquid than charge ( a ). 

Although the two charges given may work well on a silicious 
ore, it must be borne in mind that the fluxes have to be varied 
according to the gangue of the ore and the minerals contained 
therein. If the ore carries much lime or is high in metallic ox¬ 
ides, the borax should be increased or Si 0 2 added. If barite is 
present, fluor-spar, borax or silica must be added as a flux. If 
Fe2C>3 or MnC>2 is present in large amount, the reducing agent 
must be increased if we do not know the oxidizing power of the 
ore, because they will be reduced to the lower oxides (FeO and 
MnO) by the argols, and not enough argols will be left to reduce 
the necessary 24 or 28 grammes of lead. (See pages 72 and 73.) 

The following charges may make the matter a little clearer. 



Use 

E or F crucibles. 




Charge ( c ) 

(d) 

(<*') 

(e) 




\ 

Gangue = 



Fea03+ Si02- 

silicate of alu- 

Limestone in tne gangue. 

Oxidizing power = $. 


mina, and 





magnesia. 


r Ore. £ A.T. 

Ore. \ A.T. 

* A.T. 

i A.T. 

t> 

23 0) 

Bicarb, soda .. .gms. 15 

Bicarb, soda., .gms. 15 

30 

30 

^ 23 

G 'u ^ 

Borax. “ 10 

Borax. “ 10 

10 

IO 


Litharge. “ 60 

Litharge. ‘ ‘ 60 

30 

40 

s° 

Argols (R.P.="8) “ 3I 

Argols. “ 4 

6' 

3 


[ Silica (Si 0 2 )... “ 2 

Silica. “ 0 

2 

5 


rover of salt in each case. 













9 ° 


NOTES ON ASSAYING . 


Charge (c). —The ore is basic, therefore borax and silica, 
both acid fluxes, are used; the litharge is also kept high, for, 
though acting as a base, it is a good flux for limestone. 

Charge ( d ).—The ore is partly basic due to the Fe 2 0 3 , for 
which reason borax is used, and partly acid, due to the Si 0 2> 
for which reason the PbO is kept high. The ore also has an 
oxidizing power which would consume i gramme of argols in 
reducing the Fe 2 0 3 to FeO, hence the argols are raised to 4. 

Charge (d ').—In this charge the argols are used in excess, 
therefore the amount of PbO must be limited to 30 grammes. 
An excess of argols does no harm, and this is an instance of their 
advantage over charcoal; for if the latter was used in large excess, 
the charge would be infusible. Owing to the PbO being low, 
the soda and borax are high, and Si 0 2 is added, which also makes 
correct the ratio of fluxes to ore. This charge would not be 
suitable for an ore carrying copper or a metal that could be 
reduced, for the reducing agent is in excess. 

Charge (e ).—The ore here is a very refractory one w r hich re¬ 
quires high borax and considerable silica. 

Fusion in Pot-furnace —General Directions .—Fresh fuel is 
put on the fire and the crucibles are placed on this (furnace 
will hold four). Covers are put on the crucibles and the fuel 
is packed around them even with the top of the crucible, then 
the draft is put on the furnace and the contents of the crucibles 
melted slowly to avoid dusting. When the contents first fuse, 
lessen the draft and have the fusion take place quietly, not only 
to avoid having the contents boil over, but also to prevent the 
fusion from coming up on the sides and leaving particles of ore 
and lead. When all danger of boiling over has ceased, seize 
the crucibles with the tongs and rotate the charge several times 
while the crucible is in the furnace, then increase the heat and 
fuse until quiet, say 30 to 45 minutes. Rotate the crucible several 
times during the fusion. As a rule, the larger the amount of sul¬ 
phides present the longer the fusion will have to be in order to insure 
perfect decomposition of the ore. Magnetites and other refrac¬ 
tory ores require a long fusion and a high temperature. If nails 
are used (ores of Class II), the crucible should be left in the fire 
until no drops of lead are seen adhering to the nails, when they 


ASSAY OF ORES FOR SILVER. 


91 


OO 

OO 




UJ 


are raised out of the fusion. When the fusion is completed,, 
remove them and, holding them partly in the fusion, tap them 
gently to knock off any adhering drops of metal. Let the cru¬ 
cible stay in the furnsce two or three minutes longer, then 
take it out with the crucible-tongs, tap it gently 
upon the furnace, and pour the contents into a 
mould, which should have been previously coated 
with ruddle, chalk, or oil and then warmed. 

Allow plenty of time for the assay to cool, and 
then separate the slag from the button of lead, 
which should be soft and malleable. If thick cast-iron moulds', 
are used the assay cools almost immediately. Hammer the 
button into a cube. Notice the button carefully, also the slag 
and the crucible, for by so doing mistakes in the subsequent work 
may be avoided. Weigh the button on the flux-balances to the 
nearest gramme. If the button is hard and brittle, it should be 
scorified before cupelling. A red slag indicates copper oxide 
(Cu 2 0 ); if the salt cover is blue, it also indicates copper, due to 
CuS 0 4 or chloride formed with the salt. The button should 
stick slightly to the slag. A button jailing away from the slag 
indicates too great a heat or too long a fusion. If there is a matte 
between the button and the slag it indicates too short a fusion or 
imperfect decomposition. Hard buttons are due to the presence 
of copper or antimony or both. 

Brittle buttons may contain Cu, Sb, As, Zn, S, PbO, or it may be 
a rich alloy of Pb and Ag or Pb and Au. (14 grammes Au, 27 Pb, 
brittle.) 23.5 grammes Pb, 3.2 grammes Au, .3 gramme Ag, also 
brittle. The lead button, containing the precious metals, is cu 
pelled if it weighs 30 grammes or less. If it weighs over this or con¬ 
tains impurities, it should be scorified and then cupelled. Cupel 
in the usual manner. The silver button is weighed, correction, 
made for the Ag contained in the PbO used, and the result reported! 
in ounces per ton, and value per ton of 2000 lbs. av. In all 
crucible assays the object is to form a liquid slag by means of the. 
soda, borax, PbO, and other fluxes. The litharge is a splendid 
flux, but its main duty is to supply the lead and to collect the: 
precious metals in the fusion. This Pb (brought down by the: 
argols, sulpirides in the ore, or iron used in the fusion) settles as-a. 
fine spray through the fusion and collects the precious metals- 








92 


NOTES ON ASSAYING. 


Endeavor to keep the oxides of the metals, such as iron, copper, 
and manganese, in the condition of lower oxides, for the peroxides 
tend to carry Ag and Au into the slag. 

Ij base metals , such as Cu, Sb, and Zn , are present in the ore , 
high litharge and as small an amount oj reducing agent as possible 
should be used , to avoid reducing these metals. See Special 
Methods. 

Class I.—Fusion in the Muffle. (See Assay of Ores for Lead 
in regard to the manner in which the fusion in the muffle is con¬ 
ducted.) 

The fusions on ores in Class I can be made in the muffle as 
well as in the pot-furnace. In early days, in the Far West, a pot- 
furnace fired by solid fuel was used almost entirely. At 
present such a furnace is very seldom seen, most assays being 
made in the muffle. Since the gasoline and oil furnaces, 
especially the combination ones, have been introduced, one se( s 
many fusions made in these crucible-furnaces as well as in 
the muffle. The pot-furnace has the advantage over a muffle, 
fired by solid fuel, in that a much higher temperature can be ob¬ 
tained, which is very essential in the fusion of some refractory ores. 

The charge is usually made up somewhat as follows (use an 
A or B crucible, Colorado form): 

' Ore. 

Sodium bicarbonate 
Mix in the _ Borax glass. 

crucible * Litharge. 

Argols (R.P. = 7J) . 

„ Silica (Si 0 2 ). 

In this charge we aim to have fluxes sufficient to form a good 
slag and yet give a fusion which will boil up but slightly. For 
this reason the litharge is high, the soda low, and borax glass is 
used in place of borax. Iron oxides require much Si 0 2 in the 
charge, so a variation in the amount of either Si 0 2 , litharge, or 
borax glass from that given will generally make the fusion satis¬ 
factory. The following will serve as an example of an ore which 
was decomposed in a pot-fusion, but not in a muffle, heated by coke: 


2 A-T. 

15-10 grammes 
o -5 

60—90 “ 

Si “ 

1-3 “ 

Cover of salt. 









ASSAY OF ORES FOR SILVER. 


9$ 


Ore 2298-5, consisting of hematite and some quartz. 


Mix 
in the 
crucible 


■ Ore. 

Sodium bicarbonate 

Borax glass. 

Litharge. 

Silica (Si 0 2 ). 

k Argols.. 


i A.T. 

15 grammes 


. 90 “ 

• 3 “ 

• s 

Cover of salt. 


Fusion was made in a B crucible at the highest temperature 
of the muffle for 55 minutes. The result was a small lead but¬ 
ton, with the slag completely full of fine lead globules. A second 
charge was fused for i \ hours, but gave the same result. 

A charge identical in every way was then fused in a B cru¬ 
cible for 55 minutes in a pot-furnace heated by coke, and the 
result was a 44-gramme lead button and a clean slag free from 
lead globules. The explanation is that the heat was not suffi¬ 
ciently high in the muffle-furnace for the character of the charge- 

Class II. (Ores carrying sulphides, arsenides, or organic 
matter, i.e., ores which can decompose litharge with a reduction 
of lead.) —Crucible assays of ores in this class can be made by 
two methods. 

1 st Method. —In this the reducing power of the ore is first 
determined by a preliminary fusion and the regular fusion charge, 
based on this reducing power, figured out afterwards (see page 103). 

2d or Iron Method. 

Therefore before taking up the actual assay of sulphide 
and arsenical ores under this class the following experiments 
should be carefully studied and considered. They all have 
an important bearing upon the assay of these ores, and the 
experiments have been carried out from time to time as 
things came up in the laboratory which suggested them. 
A student commencing assay work seems to be under the impres¬ 
sion that the fluxes used and the amounts taken are chiefly mat¬ 
ters of guesswork. No greater mistake can be made. All the 
fluxes used have a bearing upon the work, and if the proper 
amounts are not taken, most unexpected results will follow, 
and the assays will be inaccurate for both the silver and the gold. 









94 


NOTES ON ASSAYING. 


Samples to be assayed constantly vary in composition, conse¬ 
quently the fluxes and the amounts used in the charge must 
.also vary.* 

Class II, Method I. Reducing Power of Ores.—In taking up 
this work, it seems necessary to distinguish between the working 
reducing power and the true reducing power of an ore. 

The working reducing power is that by which we can obtain 
a satisfactory lead button in the regular jusion weighing within 
i to 4 grammes of the weight calculated for. 

The true reducing power seems to be a difficult thing to 
determine. 

The working reducing power can be obtained in either of 
two ways: 

A. By the use of the same amount of sodium bicarbonate 
or carbonate as of ore used and a large excess of litharge, i.e., 
.40 to 50 times the amount of the ore. 

B. By allowing a certain amount of sodium bicarbonate or 
^carbonate to replace a given quantity of litharge. 

The following charges will serve as illustrations. 

PRELIMINARY FUSIONS. 

In our regular assay fusion, the amount of bicarbonate of 
soda used is generally the same as the ore or twice the ore; there¬ 
fore in the preliminary fusion we maintain the same ratio. 

Weigh the fluxes out first and place the ore on top, then mix all 
in the crucible. 

Method A. Use an E or F crucible. 


Take 

2 

grammes 

of 

ore if the quantity of 

sulphides 

is 

very large. 

i i 

3 

< < 

t < 

i C < < ( ( 

i i 

< < 

i < 

<< 

medium. 

i < 

5 

C ( 

< < 

< < ( ( ( < 

( i 

c < 

< < 

< < 

small. 

i C 

10 

i < 

< < 

Hit € < 

i C 

c < 

i < 

«< 

very small. 

< ( 

80 

* < 

< i 

litharge. 







“ same amount of sodium bicarbonate as of ore taken. 

“ 5 grammes glass. 

SiO„, none. Cover of salt in each case. Make one assay and fuse for 8 to 12 
minutes. 

The working reducing power of any ore (except perhaps some 
high-grade copper ores) can be obtained by some one of the 
above charges. Less litharge can be used in the case of most ores, 

* There is a very large field for research work in the analysis of slags, 
especially from crucible work. 



ASSAY OF ORES FOR SILVER. 


95 


but beginners will often obtain incorrect results by using too 
much ore and insufficient litharge or too much Si 0 2 . This last 
is therefore left out entirely. 

Method B. Use an E or F crucible. 


Take 2 grammes of ore if the quantity of sulphides is very large. 


i < 

3 

< < 

i < 

C C ( ( n 

< < 

i < 

( < 

“ medium. 

i ( 

5 

( < 

( ( 

(i a a 

< ( 

< ( 

< < 

“ small. 

< < 

10 

( ( 

( i 

a i ( i i 

< ( 

< < 

c < 

“ very small. 

< ( 

60 

< i 

( c 

litharge. 






“ 6, 6, 10, and 20 grammes of sodium bicarbonate respectively. 

“ 5 grammes of glass. 

Cover of salt. Make one assay and fuse for 10 to 15 minutes. 

This method usually gives a higher value for the R.P. of 
an ore, especially on heavy sulphuretted ones, owing to the increase 
of the bicarbonate of soda which takes the place of the extra 
20 grammes of litharge used in method A. hod. 

In finding the reducing power of an ore, use one of these 
methods. Having determined the R.P., figure out the charge 
for the regular fusion, page 103. Conduct the fusion as when 
determining the R.P. of argols and charcoal, only be still more 
careful that the contents of the crucible do not boil over. Fuse 
for ten to fifteen minutes at a high temperature, pour fusion, and 
weigh the resulting lead button upon the pulp-balances to the 
first place of decimals. 

If 5 grammes of ore were used and the lead button weighs 
5.5 grammes, the R.P. of the ore is equal to 1.1. 

The amount of lead thrown down in the preliminary fusion 
is influenced by the reagents in the charge, the relation of these 
to each other and to the ore used, also by the temperature. 

That is, an incorrect working value may very easily be ob¬ 
tained for an ore in one of the following ways: 

(a) By the use of too much silica. 

(b) By the use of borax and no sodium bicarbonate. 

(c) By the use of too little litharge or, what is the same thing, 
too much ore for a given quantity of litharge. 

(d) By omitting the sodium bicarbonate. 

(e) By an incorrect temperature. 

The following experiments, taken from the thesis of Messrs. 
M. Brown, Jr., and R. C. Reed, of the class of 1904, illustrate 
these points and are of especial value. 


NOTES ON ASSAYING. 



The ores they worked upon were grouped as follows, accord¬ 
ing to their R.P.: 

Sulphides very large, very high R.P. above 8 

large, high R.P. 4 to 8 

medium, R.P. 1 to 4 

small, R.P. below 1 

(a) The effect oj too much Si 0 2 . 


( c 


(( 


(( 


Ore No. 2420-4. 


Ore, grammes. 


3 

3 

3 

3 

3 

Litharge, “ .. 


60 

60 

60 

60 

60 

Silica, “ . 


— 

2 

4 

6 

8 

Salt. 


cover 

cover 

cover 

cover 

cover 

Time of fusion, minutes. . 


13 

10 

10 

13 

13 

Temperature, degrees C. . 


1240 

1290 

1320 

1160 

1265 

Lead, grammes. 


14.72 

13.61 

n -93 

5 - 7 ° 

3-6 3 

R.P. 


4.91 

4-54 

3-98 

1.90 

1.21 

Ore No. 2545, ZnS and Other Sulphides. 

Ore No. 605. 


Ore, grammes 5 

2 

3 

Ore, 

grammes 10 

3 

Bicarb, soda, “ 5 

2 

6 

Bicarb, soda, “ 

10 

3 

PbO, “ 100 

60 

5 ° 

Litharge, ** 

100 

120 

Si 0 2 , “ 10 

0 

0 

SiO a , 

a 

10 

3 

Cover of salt in each case. 



Cover of salt. 



Time, minutes. ... 12 

12 

12 

Fusion, 25 minutes 


Lead, grammes. . . 13 

16.66 

25.76 

Lead and matte = 

= 42.2 


R.P. 2.6 

8-33 

8.58 

Lead 

= 


24 


Working value = 8.45, i.e., the aver- Working value = 8 

age of the last two. 


In five of these fusions so much litharge combined with the 
silica to form lead silicate that not enough was left to decompose 
the ore. 

(6) The effect oj borax and no sodium bicarbonate. 


(PbS 

Ore No. 2420-4 ■< Cu 2 S 

( Si 0 2 and CaS 0 4 . 


Ore, grammes. 

3 

3 

3 

3 

3 

Bicarb, soda, “ . 

— 

— 

— 

6 

3 

PbO, “ . 


60 

60 

60 

60 

Borax, “ . 


6 

9 

— 

3 

Salt. 


cover 

cover 

cover 

cover 

Time, minutes. 


10 

10 

13 

— 

Temperature, degrees C. ... 

1265 

1200 

1225 

1330 

1345 

Lead, grammes. 


12.57 

I 3 -I 3 

23.26 

18.67 

R.P. 

... 3.80 

4.19 

4-37 

7-75 

6.22 
























ASSAY OF ORES FOR SILVER. 


97 


In this ore, owing to the galena present in it, the effect of 
borax is to increase the size of the lead button. 

(c) The use of too little litharge will give an incorrect value, 
especially if the R.P. is high; for instance, using 5 or 10 grammes 
of a heavy sulphide ore, 5 or 10 of soda, and only 60 of PbO. 

(1 d) Omitting the carbonate of soda. The effect of this reagent 
upon the size of the lead button is more marked than any other, 
and as it is always used in the regular fusion of an ore, to omit 
it in the preliminary is fatal, if the weight of the final lead button 
is to be anywhere near the amount desired qr calculated for. 

In our regular ore fusion the soda is either the same amount 
as the ore or twice the amount, and for this reason these propor¬ 
tions are maintained in the preliminary fusion. 


Silicious ore carrying FeS 2 . Sulphur = 31.34%. Through 160 sieve. 


No. of Fusion. r. 2 3 4 5 6 7 8 9 

Ore, grammes. 5 5 5 5 5 5 5 5 5 

Sodium carbonate, grammes. . — — — — — 5 20 5 40 

Litharge, ” .. 100 150 200 250 400 100 100 200 100 

Borax, “ .. — — — — — — 5 — — 

Glass, “ — — — — — — 5 — — 

Silica, “ .. — 

Cover of salt in each case. 

Lead, grammes. 25 26 27 28 39 35 38 39 39 

R.P. 5 5 - 2 5-4 5 - 6 7 *o 7 *° 7 - 6 7 - 8 7 - 8 


The R.P. of this ore is evidently 7.8, and the fusions show that 
a certain amount of carbonate of soda is equal to so much litharge. 

Let us see if it can be determined what reactions have taken 
place in the foregoing fusions (1 to 9) or when FeS 2 and PbO 
are brought in contact. 

In Mitchell’s Assaying w r e find that FeS 2 requires 50 parts 
of litharge to completely decompose it, any more than that hav¬ 
ing no effect upon the size of the lead button. Fusion 5 seems 
to require 80 parts, and this ore is not pure FeS 2 . 

1. FeS 2 +5Pb0 = Fe0+2S0 2 +5Pb, that is, 2S = 5Pb or one 
of FeS 2 will reduce 8.62 lead. 

2. 3FeS 2 +i6Pb0 = Fe 0 + Fe 2 0 3 + 6 S 0 2 +i 6 Pb. HereFeS 2 re- 
duces 9.2 lead, or 6S = i6Pb. 

3. 2FeS 2 +iiPb0 = Fe 2 0 3 +4S0 2 +iiPb. Here FeS 3 reduces 
9.48 lead, or 4S = nPb. 







9 S 


NOTES ON ASSAYING . 


The ore carried 31.34 per cent sulphur; therefore for 5 
grammes of ore in reaction No. 1 we should have 


or 


8.62X5X31.34 

53-33 


2 5 - 3 > 


5X-3i34X5Pb 


which is the amount of lead reduced in fusion No. 1. 


9.2X5X31-34 

53-33 

9-48X5X31-34 

53-33 


= 27, or the amount reduced in fusion 3. 
= 27.9, or the amount reduced in fusion 4. 


The question now arises how the 39 grammes of lead reduced 
in some of the other fusions can be accounted for. In order to 
obtain this amount of lead from reaction No. 3, the ore would 
have to contain 44.8 per cent of sulphur. The large buttons of 
lead must therefore be due to something else, and the carbonate 
of soda must be responsible for it, for a large amount of soda 
apparently takes the place of a certain amount of litharge. 

An explanation seems to be that the S 0 2 formed in the re¬ 
actions given is further oxidized to S 0 3 or forms Na 2 S 0 4 , which 
is confirmed by finding sulphates in the slag. 


Na 2 C 0 3 breaks up by heat into Na 2 0 + C 0 2 . 

Na 2 0 + S 0 2 +Pb 0 = Na 2 S 0 4 +Pb, 


or 


Na 2 C 0 3 + S 0 2 + PbO = Na 2 S 0 4 + Pb+ C 0 2 . 


In this equation 1 S = i Pb = i Na 2 C 0 3 . If the Na 2 C 0 3 is 
completely changed, we shall have = 1.95 grammes of 

2 3 

lead, and 5 grammes of soda will give 9.75 of lead. Comparing 
fusion 1 with 6, and 3 with 8, we see that the lead buttons,. in 
the fusions where soda is used, are larger by practically this 
amount. 

It is very evident from these experiments that if the ratio of 
the soda to the ore is one to one in the preliminary fusion, it must 








ASSAY OF ORES FOR SILVER. 


99 


be one to one in the regular fusion, otherwise the resulting lead 
button will be quite different from what is expected. 

The following fusion also shows how S 0 3 is formed. 

(PbS carrying about 84% lead): 

Ore.;. 1 A.T. 

1 

Bicarb, of soda. 40 grammes 

Borax. 15 “ 

Litharge. 150 “ 

Cover of salt. 

A 3 5-minute fusion gave a lead button weighing 97 grammes. 

If the reaction PbS+3Pb0 = S0 3 +4Pb takes place and the 
ore carries 84 per cent lead, then we should expect to obtain 
97 grammes of lead, because by this reaction 1 gramme of PbS 
reduces 3.46 grammes of lead and 28.28 (PbS in ore)X3.46 = 97.8. 
On the other hand, if S 0 2 was formed (2PbO+PbS = S0 2 +3Pb), 
only a 73.2-gramme lead button would be obtained. 

The following are some other tables showing the effect of soda: 


Ore 1919. Through 140. ZnS with very little gangue. 


No. of Fusion 

229 

230 

109 

no 

108 

Ore, grammes. 

3 

3 

3 

3 

3 

Bicarb, soda, “ . 


3 

6 

9 

3 

Litharge, “ . 

Cover of salt 

60 

in each 

60 

case. 

60 

100 

Time, minutes. 

15 

15 

15 

15 

15 

Temperature, degrees C. 

• 1345 

1330 

1170 

1265 

1 i 4 S 

Lead, grammes. 

,. 18.34 

21.2 

24 

24.92 

22.03 

R.P. 


7.07 

8 

8.31 

7-34 


No. 229 gives too low a result, because no soda is used. 

No. 230 “ “ “ “ “ “ with an ore having as high a R.P. as 

this ore, if the soda is the same as the ore, the ratio of litharge to ore should be 
40 or 50 to 1, whereas the ratio used is only 20 to 1. 

Comparing fusions 108 and 109 it is seen that 3 grammes 
of soda takes the place of more than 40 grammes of litharge. 

The limit of soda is apparently twice the ore, for the slight 
difference in the lead buttons in Nos. 109 and no may be 
accounted for by the difference in temperature. 

Fusion 146, on page 72, fixes both the amount of soda and 
litharge to use for that particular ore. 













IOO 


NOTES ON ASSAYING . 


Ore D. Mostly galena, a little pyrite, and a slight amount of gangue, i.e.,a 


medium amount of sulphides. 


No. of Fusion.. 182 

183 

184 

185 

186 

Ore, grammes.... 

5 

5 

5 

5 

5 

Bicarb, soda, “ _ 


5 

8 

10 

15 

Litharge, “ .... 


60 

60 

60 

60 


Cover of salt 

in each 

case. 



Time, minutes. 


10 

11 

11 

10 

Temperature, degrees C. . 

.... 1225 

1280 

1290 

1265 

1250 

Lead, grammes. 


20.68 

20.66 

20.80 

21.20 

R.P. 


4.14 

4 -i 3 

4.16 

4-25 


This ore has a much lower R.P. than the previous ones, 
and we find that the same amount of soda as ore gives prac¬ 
tically the same value as when three times the amount of soda 
is used. Where the soda is omitted the R.P., as usual, is too low. 
Owing to the medium amount of sulphides and the fact that they 
are mostly galena, 60 grammes of PbO are sufficient for 5 grammes 
of ore when 5 of soda are used. 

Ore 900. Silicious ore carrying pyrite and a little galena. Small amount 


of sulphides. 

No. of Fusion.. 

20S 

174 

209 

17s 

Ore, grammes... 


5 

5 

5 

5 

Bicarb, soda, “ 


0 

5 

8 

10 

Litharge, “ 


60 

60 

60 

60 

Time, minutes. 

Cover, of salt in each case. 

10 

10 

Temperature, degrees C.. 


1320 

1280 

1160 

1305 

Lead, grammes. 


8.82 

12.98 

13.06 

13.08 

R.P. 


1.76 

2.60 

2.61 

2.62 


This ore has a still lower R.P.; soda is still necessary, but 
here a smaller amount than 5 grammes would no doubt do for 
5 grammes of ore. 


Ore 262. Very small amount of sulphides. 


No. of Fusion.. 197 

198 

199 

200 

136 

Ore, grammes. 


10 

10 

10 

10 

Bicarb, soda, “ . 


5 

10 

i 5 

10 

Litharge, “ .. 

Cover of salt in each 

60 

case. 

60 

115 

Time, minutes. 


10 

10 

10 

20 

Temperature, degrees C.. 


1400 

1385 

i 37 o 

1280 

Lead, grammes. 


5-46 

5-40 

5 - 5 i 

6.02 

R.P. 


•55 

•54 

•55 

.60 
























ASSAY OF ORES FOR SILVER. 


roi 


Owing to the few sulphides in this ore we are obliged to use 
io grammes, and we find an instance of where nearly the cor¬ 
rect R. P. can be obtained simply with litharge. This is only 
possible owing to the very large ratio that the litharge bears to 
the sulphides. 

The foregoing tables show that when the R.P. of an ore is 
to be determined the proportions of soda and litharge to ore, 
as suggested on pages 94 and 95, should be used. 

Similar experiments with borax and borax glass show that 
its effect is, in some cases, to diminish the size of the lead but¬ 
ton, in others to increase it. On ores carrying galena, and on 
some others, the addition of borax up to a certain amount in¬ 
creases the size of the lead button. 

I prefer not to use it in the preliminary fusion. 

Ore D. See page 100. 


Ore, grammes. 

5 

5 


5 


5 

5 

Borax, “ . 

0 

3 


6 


9 

12 

Litharge, “ . 

60 

60 


60 


60 

60 


Cover of salt in each 

case. 




Time, minutes. 

10 

10 


10 


10 

10 

Temperature, deg. C.... 

1225 

1490 

1320 

1330 

1320 

Lead, grammes. 

14.82 

15.06 

15 ' 

, 11 

16.67 

16.96 

Galena, carrying 84% Pb. 








Ore, grammes. 

5 


5 


5 


5 

Borax, “ . 

— 


6 


9 


12 

Litharge, “ . 

60 


60 


60 


60 


Cover 

of salt 

in each case. 




Lead, grammes. 

I 3 -I 4 

13 - 

5 i 

14. 

83 


14.47 


The reaction, PbS+2Pb0 = S 0 2 +Pb, evidently has some¬ 
thing to do with this, or else the borax is broken up into Na 2 0 -b 
2 B 2 0 3 and the Na 2 0 acts as explained in the case of sodium 
carbonate. 

(e) Effect of Temperature .—As has been pointed out on page 86, 
the size of the lead button is influenced by the temperature. 
The experiments of Messrs. Brown and Reed seem to indicate 
that when two similar charges, containing no soda, are fused at 
different temperatures, the one having the higher temperature 
will give the larger lead button. 

When soda is present, however, an increase of temperature 
diminishes the size of the lead button. 












Arsenopyrite* 


102 NOTES ON ASSAYING. 

Pyrite. 






r - 


•% 

/ V 

Argols. . .. 

3 

3 

Charcoal. 1 

1 Ore. 

• 3 

3 

3 3 

Litharge .. 

60 

60 

60 

60 Soda. . .. 

• 3 

3 

6 6 

Glass. 

10 

10 

10 

10 Litharge. 

. 100 

100 

5° 50 




Cover of salt in every case. 




Time, min. 

i5 

13 

15 

15 

i5 

25 

15 17 

Temp., 0 C. 

935 

ii55 

935 

ii55 

1010 

1120 

980 1305 

Lead.28 

•5i 

29.84 

26.49 

27-57 

22.14 

19.00 

22.2 21.50. 


The effect of Si 0 2 , as shown on page 96, is always to diminish 
the size of the button in the preliminary fusion. This is due to 
its combining with the litharge, leaving just so much less litharge 
free. Its effect is not so marked when soda is present, for some 
silicate of soda is formed. 

The following experiments show what effect sulphides have 
on lead silicates. A singulo silicate 2Pb0,Si0 2 was made by 
fusing 296 grammes of PbO with 40 grammes of Si 0 2 . The 
resulting silicate was pulverized and used in the following fusions. 
(Pot-furnace, D crucibles.) 


ZnS (R.P.='8.3i). 

FeS 2 (R.P.= 

7.8). ,- 

PbS. 

Ore, grammes.. 

2I 

2 \ 

2* 

2* 

Lead silicate, “ 

5° 

50 

50 

50 

Bicarbonate of soda, “ 

10 

IO 

— 

2* 

Cover of salt in 

each case. 



Time of fusion, minutes. 

17 

15 

12 

15 

Temperature, degrees C. 

1340 

ii95 

1415 

1290 

Lead, grammes. 

18.97 

17.68 

3-67 

6.89 

Matte, “ . 

None 

None 

None 

None 

Per cent of the total lead that the 





ore could reduce. 

9 1 -4 

9°-7 




A trisilicate of lead (2Pb0,3Si0 2 ) was then made by fusing 
198 grammes of PbO with 80 grammes of Si 0 2 , pulverized and 
used in the following fusions. (Pot-furnace, D crucible.) 


ZnS (R.P. = 8. 3 i). FeS 2 (R.P.<= 7.8) PbS. 


Ore, grammes. 

2I 

2$ 

2 § 

Lead trisilicate, “ . 

5o 

50 

50 

Sodium bicarbonate, “ . 

2I 

2* 

H 

Cover of salt in 

each case. 



Time of fusion, minutes. 

12 

12 

13 

Temperature, degrees C. 

ii95 

1280 

1370 

Lead, grammes. 

I -5 I 

None 

1.82 

Matte, “ . 

4-45 

6.10 

• XI 























ASSAY OF ORES FOR SILVER. 


103 


These results show that sulphides will almost wholly decom¬ 
pose a singulo-lead silicate, and soda materially aids the reduction. 
A trisilicate is not decomposed, and it is assumed from this that 
silicates of lead higher in silica than the trisilicate are not decom¬ 
posed. 

This explains why the use of too much silica in a charge 
results in a matte (Ore 605, page 96), the incomplete decomposi¬ 
tion of the ore, or both, for the sulphides are unable to decompose 
the higher lead silicates with a reduction of lead, and there is not 
sufficient PbO left in the charge to decompose the sulphides. 

For experiments upon the addition of Si 0 2 to the regular 
assay of ores, see pages 106 and 107. 

Class II. Regular Fusion. (Pot-furnace.) Conduct the 
fusions as described under Class I, using especial care where 
much nitre is present in the charge. If in the preliminary fusion 
the amount of bicarbonate of soda was the same as the ore, 
figure the regular charge from the value obtained. If the soda 
was twice the ore, figure the regular charge from that value and 
keep the soda twice the ore in the regular fusion. The amount 
of PbO, nitre, and argols to be used will have to be calculated 
in the case of each and every ore. For the R.P. of argols take 
the value you find in Testing for Reducing Power, page 83. 

The following will serve as examples. 

No. 1. No. 2. No. 3a. (FeSp + PbS + SiOj.) 

Suppose the preliminary fusion on. 5 5 3 grammes ore 

gave a lead button weighing. 4.5 9-° 12 grammes 

Then the reducing power =.9 1.8 4 


Make up the charges as follows, using an E or F crucible: 


Charge.No. 1. No. 2. 

Ore (Si0 2 and FeS 2 ). h A.T. * A.T. 

Sodium bicarbonate. 15 g m - I 5 or 3° 

Borax. 5 u 5 5 

Litharge. 60 “ 75 5° 

Argols (each gm. reduces 8 gm. of Pb). .. i£ “ — — 

Nitre (KN0 3 ). — — — 

Iron. — — — 

Glass. 5 — 

or Silica (Si0 2 ). — o o 


No. 3a. No. 3&. 

i A.T. *A.T. 
15 or 18 


5 

100 


5 

60 


10 

3 


Cover of salt in each case. 

In these examples of sulphide ores the object is to decompose 













104 


NOTES ON ASSAYING. 


the ore and to obtain a lead button weighing between 25 and 30 
grammes. 

The soda is generally low, but may be high, and a little borax is 
used on account of the metallic oxides formed from the decompo¬ 
sition of the sulphides. 

Charge No. 1 .—The ore itself will reduce 13.1 grammes of 
lead; we desire a button weighing between 25 and 30 grammes; 
therefore 1^ grammes of argols (R.P. = 8) are added to the charge. 
The litharge is 60, because the soda is low. 

Charge No. 2 .—The ore itself will reduce 26.2 grammes of 
lead, so no reducing agent is necessary. 

Charge No. 3a .—The ore will reduce 14.58X4 = 58.3 grammes 
of lead; the button desired is, say, 28; lead to be oxidized, 30.3. 
One gramme of nitre oxidizes 4.3 grammes of lead or its equiva- 

lent in sulphides (see page 81); therefore = 7 grammes of 

4-3 

nitre are necessary. 

In Charges 2 and 3a the litharge is above 30 grammes, because 
we need an excess above that called for by the reducing power of 
the ore. For instance, in 3a, J A.T. of ore will reduce 58.3 
grammes of lead, and this may be obtained from 62.8 grammes 
of PbO (Pb:PbO: 158.3 :x). If we used this amount oj PbO, 
we might obtain a matte (PbS) due to some of the litharge com¬ 
bining with the Si 0 2 and the gangue in the ore, which would 
leave insufficient PbO to decompose the ore. 

If just the calculated amount of litharge is used a matte 
is less liable to form when nitre is present in the charge than 
when it is absent. 

That is, it would be less liable to form in ore x than it would 
in ore y. 


Ore (R.P. = 7). h A.T. 

Bicarb, of soda. 15 grammes 

Borax. 8 

Litharge (14.58X 7 = 102.0) no “ 

Nitre. 18 J “ 

Glass. 12 

Cover of salt. 


y 

Ore (R.P. = 2). I A.T. 

Bicarb, of soda. 15 grammes 

Borax. 8 “ 

Litharge (14.58X2 = 29.1) 31 “ 

Nitre.— 

Glass.— 

Cover of salt. 















ASSAY OF ORES FOR SILVER. 


!05 

When it does form it is liable to give low silver and gold values, 
so it is deemed advisable to use from 15% to 25% of litharge 
above what the R.P. of the ore calls for. 

The following ore will serve as an example of one giving low 
results. 


Ore 1530. Arsenopyrite and pyrite (R.P. = 7). 


Number of Fusion. 

1 

2 

3 

4 

5 

Ore. 


h A.T. 

\ A.T. 

h A.T. 

\ A.T. 

| A.T 

Bicarb, soda, grammes. 

15 

i 5 

i 5 

i 5 

15 

Borax, 

a 

8 

8 

8 

8 

8 

Litharge, 

« 

88 

99 

no 

121 

132 

Nitre, 

a 

i8£ 

i8| 

i8| 

i8| 

Hr. 

CO 

M 

Glass, 

u 

9 

10 

12 

13 

15 




Cover of salt 

in each case. 



Gold, grammes. 


.00150 

.00152 

•00155 

.00156 

.00157 

Ounces. 


3 

3-04 

3.10 

3 * 12 

3*14 


The R.P. of the ore being 7, ^ A.T. calls for 109 grammes of 
litharge; therefore we find that the amount of litharge used was 
as follows: 


10% Calcu- 10% 20% in' 

less than lated in excess excess 

the required amount 

amount 

The nitre in these fusions not only oxidizes the sulphides, 
but also the lead, as it is thrown down in a fine condition, prob¬ 
ably in this way: 

2KN 0 3 = K 2 0 + N 2 0 8 ; 

N 2 0 s = 2 N 0 -f- 3 0 ; 

3 0 + 3 Pb = 3 PbO. 

Charge jb is made up by decreasing the litharge and increas¬ 
ing the soda, i.e., a certain amount of soda takes the place of 
litharge in decomposing the ore. No doubt there is some definite 
ratio between them, so that when the soda is increased the litharge 
can be decreased. Whether this ratio will apply to every sul¬ 
phide ore is doubtful. 

In using this method care must be taken that the soda is kept 
sufficiently high and that sufficient litharge is used, for otherwise 
the lead buttons will be too small or a matte may result. 

It seems advisable to use not less soda than ore in any fusion, 


20 % 
less than 
the required 
amount 





io6 


NOTES ON ASSAYING. 


nor less litharge than 70 or 80 grammes for A.T. of ore when 
the soda is low and the R.P. is 4 or over. 

If an ore carries a very small amount of sulphides and has a 
reducing power of, say, .2, do not make up a charge as follows: 

Ore. J A.T. 

Sodium bicarbonate.30 grammes 

Borax. 5 “ 

Litharge. 30 “ 

Argols (R.P. = 10). 3 “ 

Cover of salt. 

This is incorrect, for the reason that the argols may reduce 
all the lead from the litharge before the sulphides in the ore are 
decomposed by the PbO. Whether the ore carries a small or a 
large amount of sulphides, unless iron is present , some excess of 
PbO must be left in the fusion to make sure that all sulphides 
are decomposed. 

The following charge would be correct and the reducing power 
would not have to be determined: 


o 

Uh <u 
•*—» ( 

£ £ 
• r-H O 

x 2 
§ 0 



Ore.. 

Sodium bicarbonate. 

- Borax. 

Litharge. 

Argols (each -gm. reduces 10 gm. of Pb) 


h A.T. 


30 grammes 


a 


5 

40 
2 

Cover of salt. 


u 


Addition of Si 0 2 to a Fusion.—It has been shown on page 96 
that the addition of too much Si 0 2 to a charge will result in a 
small lead button and an incorrect R.P. 

If too much is added to a regular fusion, the lead button will 
be too small or smaller than that calculated for, and the ore may 
not be wholly decomposed. 

When the R.P. of an ore is low it necessarily follows that the 
sulphides must be small and the gangue consequently large. 
Examples Nos. 1 and 2, given on page 103, are of this character, 
and as 60 and 75 grammes of litharge are used respectively, no 














ASSAY OF ORES FOR SILVER. 


107 


Si 0 2 is added, because the ores have sufficient gangue to prevent 
the PbO cutting into the crucibles. In example 3a the ore has a 
R.P. of 4; therefore the gangue is not large, and as 100 grammes 
of PbO are used, 3 grammes of Si 0 2 are added. 

When the PbO is high a larger amount of Si 0 2 is necessary 
than when the PbO is low; then, again, the character of some ores 
admits the addition of a much larger amount of Si 0 2 to a given 
quantity of PbO than another ore when the same amount of 
PbO is used. 

The amount of soda in the fusion also has an important bear¬ 
ing upon the question. 

Just how much Si 0 2 to add is an important question, and 
the following experiments were carried out by Messrs. Brown 
and Reed to see whether some definite rule could be established: 


Ore 900. R.P. = 2.6, page 100. 


No. of Fusion. 

180 

181 

234 

250 

251 

252 

Ore . 

i A.T. 

4 A.T. 

\ A.T. 

4 A.T. 

4 A.T. 

4 A.T. 

Bicarb, soda, grammes. .. . 

15 

30 

15 

15 

15 

15 

Litharge, “ . 

75 

5 ° 

60 

60 

60 

60 

Nitre, “ . 

3 

3 

2^ 

24 

24 

24 

Silica, “ . 


— — 4 

Cover of salt in each case. 

8 

16 

Time of fusion, minutes. . .. 

25 

25 

20 

20 

21 

22 

Temperature, deg. C. 

1360 

1305 

1305 

1210 

1280 

1330 

Lead, grammes. 

22.97 

23-7 

24.6 

22 . 7 

19.9 

12.6 


The lead button from No. 250 is smaller than that from No. 
234, showing the effect of 4 grammes Si 0 2 . In 251 there was 
some matte. 

On panning this ore, the gangue in J A.T. was found to be 
10.3 grammes; if we add this amount of gangue to the Si 0 2 added 
and compare it with the soda we find the ratio as follows: 


No. 250 

25 1 

252 


1 soda : .98 silica 


1 

1 


“ : 1.22 “ 


<( 


: i -75 


(C 


Ore, No. 231. Mostly pyrite. R.P. 


No. of Fusion. 253 

Ore. 4 A.T. 

Bicarb, soda, grammes. 15 

Litharge, “ 62 

Nitre, “ 8 

Silica, “ 0 


4. Gangue, 8.1 grammes in i/ 2 A.T. 


242 

243 

244 

24s 

4 A.T. 

4 A.T. 

4 A.T. 

4 A.T. 

15 

i5 

i5 

i5 

62 

62 

62 

62 

8 

8 

8 

8 

6 

12 

18 

20 

















io8 NOTES ON ASSAYING . 



Cover of salt. 



Time, minutes. 


20 

20 

20 

Temperature, degrees C. ... 

•• 1345 

1330 

1200 

1320 

Lead, grammes. 

.. 22.5 

20.6 

14.1 

9.8 


Fusion No. 242 gave a very little matte besides the lead. 

Fusions, Nos. 243, 244, and 245 gave a steadily increasing 
quantity of matte. 

Ratio of soda to Si 0 2 added+gangue in ore= 1 to .96 in 242. 

“ “ “ “ “ “ + “ “ “ =1 to 1.37 in .243. 

62 grammes of litharge were used, because this is the exact 
amount which should decompose the ore if the R.P. is 4 and the 
other fluxes had no influence. 

If we look back to page 69, we find that the ratio of Si 0 2 to 
soda could not be over 1.7 to 1 or the fusions would be thick 
when silica, soda, and litharge were fused together, and in these 
fusions 60 grammes of PbO were free to combine with the Si 0 2 and 
make the fusions liquid, no ore being present. On page 67 
fusions show that when soda and silica are fused together the ratio 
should be 2 to 1. 

Judging from the last two tables, when J A.T. of ore, 15 
grammes soda and 60 grammes litharge are used, the ratio of 
silica (gangue in ore and silica added) to soda should not be over 
1 to 1. 

This certainly seems a safe ratio for most ores. 

In order to apply this, pan some of the ore and make an esti¬ 
mate of the amount of gangue in it. One will find that after a 
short time a very close estimate can be made, then judge the 
amount of Si 0 2 to add, taking also into consideration the character 
of the sulphides and the amount of litharge used. Some ores on 
the other hand require an unusual amount of Si 0 2 to be added. 


Ore 1919 through 140. ZnS with practically no gangue. In this instance the 
silica combines with the zinc, forming zinc silicate. 


No. of Fusion. . . 

125 

225 

226 

241 

126 

227 

228 

24S 

236 

246 

237 

Ore. A.T. . . 


* 

* 

* 

* 

* 

* 

* 

* 

* 

* 

* 

Bicarb, soda, grammes. 

15 

15 

15 

IS 

20 

20 

20 

20 

20 

20 

30 

Litharge, 


150 

ISO 

150 

150 

110 

IIO 

no 

110 

no 

no 

70 

Borax, ' 


IS 

0 

O 

0 

O 

0 

0 

6 

0 

0 

0 

Nitre, 

•« 

22 

22 

22 

22 

22 

22 

22 

22 

22 

22 

22 

Silica, 

M 

4 

8 

12 

14 

2 

4 

8 

8 

14 

30 

0 


Cover of salt in all fusions. 


Time minutes. 

30 

35 

33 

35 

30 

35 

32 

35 

35 

35 

35 

Temperature, degrees C. 

1185 

1385 

1305 

1305 

1370 

1305 

1360 


1305 


1320 

Lead, grammes. 


26.4 

28.1 

27.6 

24.9 

26.4 

28 

34.30 

28.5 

21 

24 5 
















































ASSAY OF ORES FOR SILVER. 


109 


All these fusions were perfectly liquid and there was no sign 
of a matte. Attention is called to the increase in size of the lead 
buttons when borax is used on this ore (fusions 125 and 245). 

It is seen from these fusions that, if the working reducing 
power of an ore is obtained correctly, and the fluxes in the regular 
fusion used in the proper ratio, the resulting lead button will 
come out almost exactly as calculated. 

The following show how close they often come: 


Ore. I A.T. \ A.T. * A.T. * A.T. 

Bicarb, soda, grammes. 15 15 15 15 

Borax, “ . o o o 7 

Litharge, “ > 65 65 65 65 

Nitre, “ . 9 9 9 9 

Si 0 2 , “ . 2 4 7 7 

Cover of salt in each case. 

Time, minutes. 20 20 20 20 

Lead, grammes. 26.1 26.5 26.9 26.1 


Class II. Iron Method. (See also page 133.)—In this method 
the NaHC 0 3 or Na 2 C 0 3 must be two or more times the ore used. 

Litharge must not be over 30 grammes. 

An excess of iron must be present. 

The fusion is conducted as described under Class I. 

1 

This, in my experience, has proved a most excellent method 
on ores which do not contain arsenic, antimony, or copper. It 
saves a preliminary fusion, and a lead button of the proper size 
for cupelling can always be obtained. 

With ores containing antimony or copper a large amount of 
litharge is necessary, in order to oxidize these impurities and 
either volatilize or slag them; for this reason the iron method, 
in which only a small amount of litharge can be used, is not 
recommended. 

Arsenical ores can be assayed by this method, but special pre¬ 
cautions have to be used and they will be taken up later. The 
ore under example 3a (page 103), which has a R.P. of 4, may be 
also assayed with the following charge. 










I IO 


NOTES ON ASSAYING. 


Ore. i A.T. (FeS 2 +PbS + Si 0 2 .) 

Bicarb, soda. . 30 grammes ( always twice the ore at least) 

Borax. 8 “ 

Litharge.30 “ 

Argols.none 

Nitre.none 

Silica. 2 grammes 

or Glass .... 10 “ 

One spike or nails (twentypenny) 3, point down, each of which 

weighs about 17 grammes. 

Cover of salt. 

In this method an ore, like the one just given, is decomposed 
partly by the litharge and partly by the iron as follows: 

FeS 2 + 5PbO = 5Pb+ 2 S 0 2 + FeO; 

FeS 2 +Fe = 2FeS; 

PbS+Fe = FeS+Pb, 

PbS+ 2 Pb 0 = 3 Pb+S 0 2 . 

We also have the iron acting on the litharge and lead silicates 
that may be formed or that are present in the fusion: 

PbO + Fe = FeO + Pb; 

2Pb0,Si0 2 + 2Fe = 2Fe0,Si0 2 + 2Pb. 

The ore will be perfectly decomposed and the resulting lead 
button will weigh between 25 and 28 grammes, provided the 
ore contains no lead minerals. Practically all the lead com¬ 
pounds in the fusion will be reduced to metallic lead, and it is 
owing to this fact that the litharge must not be over 30 grammes, 
if we wish to avoid scorifying the resulting button. If an ore 
carries, for instance, 50 per cent of lead in the form of galena, 
either less than J A.T. of ore must be used or the charge made 
up in exactly the same way, and the litharge diminished from 30 
to 20 grammes. 

The resulting button from this fusion will weigh about 25 
grammes, 18 grammes coming from the litharge and 7 from the 
galena in the ore. 










ASSAY OF ORES FOR SILVER. 


Ill 


Students always ask the question, “When is iron necessary 
in a fusion?” The answer is, when 30 grammes of litharge will 
not decompose the ore taken and leave some litharge in excess. 
For instance, take the two following ores: 


(e) 


Ore (FeS 2 ). 

Bicarb, soda,. 

Borax. 

Litharge. 

Argols. 

Silica. 

Iron nails (2openny) 
Cover of salt. 
Lead button. 


\ A.T.(R.P. = 2j) 
30 grammes 

30 “ 

none 

2 ~3 “ 

3 

27 


(/) 


Ore (FeS 2 ). . 
Bicarb, soda 

Borax. 

Litharge.. . . 

Argols. 

Iron. 


h A.T. (R.P.= i£) 
30 grammes 
8 

30 “ 

none 

none 


Cover of salt. 

Lead button.22 


In (e) 30 grammes of litharge will not decompose the ore and 
leave any excess, so we put in iron. If we leave out the iron, 
a lead button and a lead matte will be the result. 

In (/) 30 grammes of litharge will decompose the ore and 
leave a little litharge in excess, so we need no iron. 

If an ore is just on the line, as one might say, for instance, 
if it has a R.P. of 2 and carries no lead, then either of the following 
charges would be correct: 


Ore. 

i A.T. (R.P. = 2 

Bicarb, soda. 

30 grammes 

Borax. 

8 

Litharge. 

30 

Argols. 

none 

Silica. 

2-3 “ 

Iron nails(2openny).. 
Cover of salt. 

3 “ 

Lead button. 

27 


Ore. § A.T. (R.P. = 2) 

Bicarb, soda. 30 grammes 

Borax. 8 

Litharge.40 to 50 “ 

Argols. none 

Iron. none 

Cover of salt. 

Lead button. 29 


If, in the last four examples given, 1 A.T. of ore was taken 
instead of J A.T., then iron would be necessary in all four cases, 
the soda would have to be 60 and the litharge 30 grammes. 
If we took 1 A.T. of a galena ore, carrying 50 per cent of lead, 
the soda would be 60 and the litharge 15 grammes. 

Many assayers object to the use of iron, claiming that it is 
liable to form a matte with consequent inaccurate results. A 
matte will never be formed if an excess of alkali flux is used, 
for any iron matte formed will dissolve in the alkaline slag. 
The following experiments show what takes place in two fusions, 
wherein there is a sufficiency of alkaline flux in one case and 
an insufficiency in the other. 
































112 


NOTES ON ASSAYING. 


The ore in each case consisted largely of FeS 2 and contained 
39.56 per cent of sulphur. The reducing power was about 8. 
Each charge was fused 40 minutes. 


(*) 


Ore. 

1 A.T 

Bicarb, soda. 

30 gm. 

C.P. litharge. 

30 “ 

Glass. 

15 “ 

Iron nails (2openny)... 

4 “ 

Borax glass cover. 

10 “ 


60 


Ore. 

i AT 

Bicarb, soda. 

3 ° g m - 

C.P. litharge. 

30 “ 

Glass. 

i 5 “ 

Iron nails (2openny).. 

4 “ 

Borax glass cover. 

10 “ 


The following results were obtained: 


Slag. 60 grammes 

Matte (FeS, a little PbS 

and alkaline sulphide).. 2si “ 

Lead. 24^ “ 

Crucible and iron and flux 

before fusion.685 “ 

after fusion.665 


Loss. 20 

Iron nails before fusion... 64 

“ “ after fusion. ... 43 


Loss of iron. 21 


Slag. 

65 grammes 

Matte. 

none 


Lead. 

26 i 

< C 

Crucible and iron and flux 



before fusion. 

662 

( i 

after fusion. 

642 

i i 

Loss. 

20 

tc 

Iron nails before fusion. . . 

63 

< t 

“ “ after fusion. 

49 


Loss of iron. 

14 

< i 


The reason no matte was obtained in (y) was owing to the 
ratio of the soda to the ore taken. 

If the soda in (x) was increased to 60 grammes, no matte 
would be obtained. The slag from (x) contained 6.73 per cent 
of sulphur, practically all as sulphide; that from (y) contained 
7.63 per cent of sulphur, practically all as sulphide. When 
the slag was treated with HC 1 , in both cases it gave off H 2 S 
strongly. A large percentage was soluble in water. In fusion 
(x) about 8 per cent and in fusion (y) about 14 per cent of the 
sulphur disappeared, probably as S 0 2 ; the remainder was 
undoubtedly combined with the iron as sulphide of iron and as 
a double sulphide of iron and soda, which was held in solution 
by the large excess of the alkali flux used. Some of this double 
sulphide of the iron and alkali was in the matte in fusion (x ) 9 
for on standing some months this matte fell to pieces. 

If arsenic had been present in this ore, an iron speiss would 


































ASSAY OF ORES FOR SILVER. 


H3 

have resulted from both fusions, with the charges given, unless- 
great care had been taken with the temperature at which the 
fusions were conducted. This question is taken up under the 
assay of ores for gold. 

It will be noticed, in all the fusions so far given, that iron 
and nitre are never used in the same charge; in other words, if 
the R.P. of an ore is determined, the charge is made up as 
described on page 103. If this preliminary fusion is not made 
and the iron method is used, then the nitre is left out. 

This raises the question of whether iron and nitre can or 
should be used in the same fusion. In the rush of a busy assay 
office there is not time to determine the reducing power of each 
sulphide ore, so in the case of ores where arsenic is present or 
suspected an assayer will use both iron and nitre in the same 
fusion. Personally I do not believe in this. An assayer in 
the West writes me: “On heavy iron concentrates, analyzing 
about 40% sulphur, 36% iron, and 10% Si 0 2 , with small amounts 
of arsenic, antimony, zinc, and lead, I use the following: 


X 

2 




Ore.£ A.T. 

Litharge.30 grammes 

Silica. 3 

Nitre. 4 “ 

Flour. 

10-gm. crucible, about f full of a mixed flux. 

Four tenpenny nails and a cover of flux. 


Sodium bicarbonate. ... 
Potassium carbonate. .. 
Borax glass. 


1 part 

1 “ 

2 parts 


For lighter sulphides the nitre is cut down, still using enough 
flour for a reducing agent.” 

Whether an iron speiss will result from a fusion like the 
above depends upon: 

1 st. The percentage of arsenic in the ore. 

2d. The temperature at which the fusion is conducted. 

3d. The amount of alkali, i.e., soda or potash, in the charge. 
If very little arsenic is present in the ore, no speiss may 
result, even with so small an amount of nitre as 4 grammes; 
but if an ore is highly arsenical, a speiss will be very liable to 

form. 










NOTES ON ASSAYING. 


114 


For the use of iron with arsenical ores and experiments thereon, 
see Assay of Ores for Gold, pages 137 to 139. 

The great advantage of the iron method is that we are always 
sure of obtaining a lead button of the proper size for cupellation 
and nitre is not used in the fusion. This appears to be a very 
strong point in its favor, especially in the assay of ores for silver, 
for I am confident that in the case of certain ores the use of much 
nitre in the fusion is the cause of low results. 

The following fusions will illustrate my meaning. 


Ore (R.P. 4.8). 

h A.T. 

\ A.T. 

Bicarb, of soda. 

15 grammes 

30 grammes 

Borax. 

5 

8 

Litharge. 

90 “ 

30 

Silica. 

3 “ 

3 

Nitre. 

11 

Iron nails 4 

Cover of salt. 


Cover of salt. 

Silver and gold. 

28.8 oz. 

3 1 - 43 oz. 

(a) 


(*) 

Ore was PbS and ZnS. 

Ore. 

. i 


Ore (R.P. = 7). \ A.T. 

Bicarbonate of soda. 15 grammes 

Borax. 10 

Litharge. 140 

Nitre. 20 

SiO,. 6 


< i 
t ( 


Cover of salt. 

Silver and gold. 63.2 oz. 


A.T. 


Bicarbonate of soda. 30 grammes 

Borax. 10 “ 

Litharge. 25 

S1O2. 3 

Iron nails (2openny). ... 4 “ 

Cover of salt. 

Silver and gold. 67.1 oz. 


i < 


For this reason it seems advisable to avoid the use 0j nitre 
in the assay oj ores jor silver , and therefore I recommend, when 
ores have a high R.P. and the iron method can he used , a charge 
like ( b ) rather than one like (a). If in (a) we took only Vio A.T. 
of ore and made up a charge on that basis no nitre would be 
needed, but this would do away with the advantage of the cru¬ 
cible assay which enables us to use large amounts of ore. When, 
however, the R.P. is not high the use of nitre may be avoided 
by taking such an amount of ore and no more as will give us 
a lead button of just the size desired. For instance, 3 /io A.T. 
could be taken where an ore has a R.P. of 3. 

In any case, whether in assaying ores for silver or for gold 
or for both, if the ores contain sulphurets we must either have 






















ASSAY OF ORES FOR SILVER. 


115 


an excess of iron present or an excess of an oxidizing agent, 
for otherwise the silver and gold may remain in the slag as a 
double sulphide of the metal to be determined and the alkali 
used as a flux. 

Large amounts of alkali or carbonate tend to carry sulphur 
and arsenic into the slag, and they will remain there in com¬ 
bination with the alkali or carbonate if the heat is kept low. 
If the heat is high, they will tend to be removed, especially if 
iron is present in the fusion. (See fusions on page 138.) 

Class II. Fusion in the Muffle.—With ores of this class the 
fusions can, in many cases, be made in the muffle, but it must 
be borne in mind that, owing to the use of nitre in one method 
and the use of high soda in the method in which iron is used, the 
fusions are very liable to boil over. 

Effect of Temperature.—In the case of certain sulphide ores, 
as in the case of certain oxide ores of Class I, it is very difficult 
to obtain sufficient heat in a muffle-furnace, fired by coke, to 
make a good fusion and have the slag free from lead. The same 
fusion carried on in a pot-furnace, heated by coke, gives perfectly 
satisfactory results, so it must be only a question as to temperature. 


Ore 255. Pyrite and a very little chalcopyrite and galena in a quartz gangue. 


R.P. = 2.7. 



Ore.. 

Sodium bicarbonate 

Borax glass. 

Litharge. 

Nitre. 

Glass. 


No. 1. 

\ A.T. 

10 grammes 


8 

100 

2 


t < 

(1 

c ( 


10 

Cover of salt. 


No. 2. 

\ A.T. 

10 grammes 
8 

too 

2 

10 “ 

Cover of salt. 


o 

p- 


0 ) 


o 

p 

pq 


o 

u 

a 

p 

U 

3 


Fusion No. 1 was made in the muffle at its highest tempera¬ 
ture, and it was very hot, for 40 minutes. The resulting lead 
button, weighing 27 grammes, dropped away from the slag, show¬ 
ing that the fusion had been too long. The button was brittle 
on top, indicating a little matte and the slag had some lead in it. 

Fusion No. 2 was for 40 minutes in a pot-furnace and every¬ 
thing was satisfactory. The lead button weighed 25 grammes, 
was soft and malleable, and no matte was present. The slag was 
perfectly free from lead. 










NOTES ON ASSAYING . 


116 


Effect on Size of Lead Button. 



A. 

B. 

C. 

Ore. 

h A.T. 

\ A.T. 

\ A.T. 

Bicarb.soda.... 

10 grammes 

10 grammes 

10 grammes 

Borax glass. 

10 

10 “ 

10 “ 

Litharge. 

100 “ 

100 

100 “ 

Nitre. 

11.8 “ 

V* 

V* 

CO 

w 

M 

11.8 “ 

Silica. 

2 

2 

2 

Salt 

cover 

cover 

cover 

Fusion. 

50 min. 

50 min. 

50 min. 

Lead. 

19.2 grammes 

22.9 grammes 

26.4 grammes 


A was in the front part of the muffle and the coolest. 

B “ “ “ middle “ “ “ “ “ hotter than A. 

C “ “ “ back “ “ “ “ “ the hottest. 


Ores Containing Organic Matter. — Ores carrying much 
organic matter, graphitic shale or graphite may cause much 
trouble in crucible fusions. Their presence is indicated by the 
fusion pufhng up, a crust forming on top with flames burning 
over it and the charge pouring thick and pasty. 

Substances of this character are not adapted to the iron 
method and should have the reducing power determined as in 
the case of sulphide ores. If the R.P. is low, a fusion can be 
made as in the case of these ores. If the R.P. is very high and 
the substance poor in silver, it is better to roast it first (see page 132) 
and then fuse it, for by so doing a large amount can be used. 
If the R.P. is high and the substance fairly rich, an assay can 
be made as in the following instance: 

Residue from a zinc retort. (R.P. = 12.) 


Ore. 3 /io A.T. 

Bicarb, soda, grammes.. 15 

Borax glass, “ 10 

Litharge, “ 130 

Nitre, “ 20 

Silica, “ 4 


Cover of salt. 

A fusion of this sort boils violently, owing to the presence 
of the nitre and organic matter and great care must be used 
in conducting it. 

Size of Lead Buttons. —In all the crucible work it has been 

advised to have the resulting lead, button weigh between 25 and 














ASSAY OF ORES FOR SILVER. 


117 

30 grammes. The reason for this is that a button of this size is 
more likely to collect all the precious metals than a button of a 
smaller size. I do not mean by this that small buttons or buttons 
up to 18 grammes may not collect all the silver and gold, for they 
can and do in most cases. But in many cases they do not, and 
the following will serve as examples. In all these fusions every 
endeavor was made to keep everything connected with the differ¬ 
ent fusions as nearly identical as possible except the size of the 
lead button. 

Ore carrying AgCl. (R.P.= i.3.) 

Lead button, grammes. 3 7 21 31 

Silver and gold, ounces. 1155.6 1217.6 1247.8 1254.2 

Two assays on an ore as rich as this may perhaps vary the 
amount that the last two assays disagree: 

Ore No. 144. 


Lead button, grammes. 

81 

ni 

23 

30 

Silver and gold, ounces. 

38.79 

42.7 

53-62 

55-7 

Ore No. 207. (R.P. = i£.) 





Lead button, grammes. 

8 

;6 

19 

28 

Gold, ounces. 

1.1 

i-37 

1.4 


Ore No. no. (R.P. less than i.) 





Lead button, grammes. 

9 

13 

25 


Silver and gold, ounces. 

1.1 

1-7 

1.8 



The loss in cupelling lead buttons weighing from 25 to 30 
grammes is no doubt slightly larger than in cupelling those weigh¬ 
ing from 15 to 20, but the loss is nothing like the difference 
shown in the foregoing examples by having the lead buttons of 
insufficient size to collect all the precious metals in a fusion. 

It will generally be observed that when especially nice work 
is being carried on, the lead buttons will weigh between 25 and 
30 grammes. 

Dusting of Ores.—Certain ores, when commencing to fuse 
in a crucible, and sometimes even before, have a tendency to dust. 
This can be easily seen by noticing the cover of salt as well as the 
rim and cover of the crucible, which will be covered with the 
fine ore blown up from little holes in the charge. Serious losses 
may occur in this way, and in the case of certain ores it is very 
difficult to account for the phenomenon. 










NOTES ON ASSAYING. 


118 


The following precautions may in many instances prevent it: 

1. After having placed the crucible in the fire, on no account 
touch or disturb it until the contents have fused or sintered. 

2. Placing a heavy cover of borax glass on top of the charge. 

3. Making a very quick fusion. 


SPECIAL METHODS. 


Silver in Copper Ores. Crucible Fusion.—Ores and products 
which contain a high percentage of a metal like copper, anti¬ 
mony, or any metal which is liable to be reduced and pass into 
the lead button in the crucible assay, are generally assayed by 
scorification or some wet process. Some ores carrying up to 
25 or 30 per cent copper can be assayed satisfactorily by the 
crucible method. If the percentage is above this and as much 
as \ A.T. of ore is taken, it is difficult to prevent so much copper 
going into the lead button that a scorification of the button is 
unavoidable. 


To test an ore for copper, boil a little of it in HN 0 3 or aqua 
regia, cool, and make strongly alkaline with ammonia. A 
deep blue color indicates the presence of copper. Much nickel 
may give a color somewhat similar. 

The following fusions made on an ore carrying 12J per cent 
of copper and consisting of pyrite, pyrrhotite, and chalcopyrite, 
with a R.P. of 5J, will show the method to follow: 


Pot-furnace, G crucible. 

Ore. \ A.T. 

Sodium bicarbonate. 20 grammes 

Borax glass. io “ 

Litharge.150 “ 

Nitre. 13 “ 

Silica. 6 “ 

Cover of salt. 
Fusion. 50 minutes 

Lead (from both fu¬ 
sions cupelled di¬ 
rectly) . 23 grammes 


Ag. 2.42 ounces 

Au.26 “ 


h A.T. 


20 grammes 

( t 


IO 

150 

13 

6 


< 4 

< i 

< < 


Cover of salt. 

50 minutes at rather low tem¬ 
perature 


25 grammes (quite soft) 
Cupels a little dark, indicating 
a little copper oxide 
2.64 ounces 
.26 “ 











ASSAY OF ORES FOR SILVER. 


119 

It will b£ seen that the object in these charges was to have 
the litharge extremely high in order to oxidize the copper and 
drive it into the slag. 

The silica was also kept high, that it might assist in slagging 
the copper. After pouring, the top of both fusions was very 
blue, indicating sulphate of copper and some chloride of copper 
in the cover of salt. * 

The slag itself was deep red, due to the Cu 2 0 carried there by 
the litharge. 

Copper mattes may be assayed in this manner using J A.T. 
or T s 7 A.T. This is a larger amount than can safely be used 
in a scorifier, which is certainly an advantage if the matte carries 
only a little silver. Then again these amounts will often bring 
down a lead button of just the desired size, so that the use of 
nitre is avoided. 

If the ore is an oxide ore, either native or due to the roasting 
of a sulphide, and contains copper, the charge is made up as 
follows and can be done either in the muffle- or the pot-furnace. 
The following ore is considered as having no oxidizing power: 


r Ore. 


i A.T. 

Sodium bicarbonate. 

. 5 

<< 

15 grammes 

Borax. 


u 

5 

Litharge. 


ii 

90-110 “ 

Argols (R.P. 10). 

. 

a 

2\ 

. Silica. 


u 

3-5 


Cover of salt. Cover of salt. 


The temperature should be medium. 

If the ore has an oxidizing power, then this must be deter¬ 
mined; for if too much reducing agent is used, the lead button 
will be too large and will probably contain considerable copper 
brought down at the same time as the lead. 

If the resulting lead button is hard or brittle, add sufficient 
lead to make the weight 60 grammes and scorify in a 2 f" or 3" 
scorifier. As soon as the lead begins to drive add a little fine 
silica and scorify at a low temperature, as described under Copper 
Mattes, Scorification Assay. 









120 


NOTES ON ASSAYING. 


The following interesting data in regard to the crucible assay 
of a cupriferous silver and gold ore were obtained by Mr. W. W. 
Trowbridge of the class of 1904. The ore was chiefly chalco- 
pyrite and showed on analysis: 


Copper. 
Sulphur, 
Lead... 
Silver... 
Gold. .. 


24% 

35 - 5 % 

3 - 5 % 
51.6 oz. 

3.1 oz. 


If we satisfy the copper with sulphur to form Cu 2 S and the 
remaining sulphur with iron to form Fe 2 S 3 , the iron would be 
34.37%, leaving 2.63% for the gangue matter. 


R.P. OF THE ORE, SHOWING EFFECT OF DIFFERENT REAGENTS. 


No. of Fusion. . 

1 

2 

3 

4 

5 

6 

7 

8 

10 

I I 

12 

19 

20 

21 

Ore, grammes. 

3 

3 

3 

3 

3 

3 

3 

3 

3 

2 

2 

2 

2 

2 

Soda, “ . 

— 

— 

— 

— 

3 

3 

3 

3 

3 

2 

2 

5 

5 

0 

Borax, “ . 

Litharge, “ . 

60 

60 

90 

90 

60 

90 

90 

120 

90 

90 

90 

40 

60 

106 

Nitre, “ . 

Silica, “ . 

— 

— 

3 

— 

— 

3 

3 

3 

— 

1 

1 

— 

— 

— 


Cover of salt in each case. 


Time fusion, min. . . 

24 

12 

14 

1 2 

15 

15 

15 

15 

10 

IS 

IS 

12 

14 

10 

Temp., deg. C. 

680 

870 

860 

1040 

910 

760 

QOO 

660 

— 



1 270 

1340 

1010 

Lead, grammes. 

9.2 

12.8 

13-4 

14 

19.8 

19.7 

19-5 

21.5 

23.5 

17 

18 

18.6 

18 

14.7 


Appearance of button: fusions i to io, brittle; n, 12, 19, 20, and 21 good. 

Slag: fusions x to 4, black; 5 to 8, dark brown; 10 to 12, black; 19, 20, reddish; 21 
black. 

Salt: fusions x to 12, green; 19, brown; 20, yellow; 21, green. 


Depending upon the reagents used, the R.P. varies from 3.06 
to 9, and the latter is considered the working R.P. 

The fusions again show how absolutely essential is the pres¬ 
ence of sodium carbonate. 


















































ASSAY OF ORES FOR SILVER . 


I 2 I 


REGULAR ASSAYS, SHOWING THE EFFECT OF DIFFERENT RF- 


AGENTS UPON 

THE 

SLAGGING 

OF 

THE 

COPPER. 


No. of Fusion. 

18 

28 

17 

35 

36 

15 

16 

25 

30 

Ore grms.... 

3 

3 

10 

IO 

10 

10 

10 

10 

10 

Bicarb, soda “ 

5 

5 

13 

13 

13 

10 

10 

10 

10 

Borax “ ... 

Litharge t: 

50 

50 

70 

70 

70 

no 

no 

no 

no 

Nitre “ ... 

4 

4 

14 

14 

14 

14 

14 

14 

14 

Silica “ ... 

— 

— 

— 

— 

— 

i -5 

i -5 

i -5 

i -5 

Salt.. 

cover 

cover 

cover 

cover 

cover 

cover 

cover 

cover 

cover 

Time of fusion, min. . 

15 

14 

16 

high 15 
20 , 6 0 

low 10 

25 

27 

20 

23 

Temperature, deg. C. 

770 

mo 

1000 

1090 

1400 

1210 

1500 

1060 

820 

1500 

Color of salt. 

pink 

red 

pink 

reddish brown yellow yellow 

— 

— 

Weight of lead. 

10.6 

10.6 

26.3 

28.5 

23-4 

25-9 

26.9 

26.2 

26.7 

Gold, ounces. 

3-09 

— 

3.09 3.09 

2.92 — 

— 

3-09 

3-°9 

Silver, “ . 

40.5 

5 o -47 

51.64 50.3 

47-3 

5 1 • 1 

50.3 

50-4 

5 i -2 

Total copper slagged,^ 

£ 52.7 7 1 -5 

33-5 

27.7 

26.6 

49.4 

48.6 

5 i 

52-5 


Ratio of PbO to the 
copper in the ore... 


69:1 


29:1 


46:1 


No. of Fusion. 

Ore, grms. 

Bicarb, soda “ . 

Borax “ . 

Litharge “ . 

Nitre “ . 

Silica “ . 

Salt. 

Time of fusion, min. 

Temperature, degrees C.. 

Color of salt. 

Weight of lead. 

Gold ounces. 

Silver “ . 

Total copper slagged, % 
Ratio of PbO to the cop¬ 
per in the ore. 


34 

41 

44 

39 

40 

22 

32 

33 

10 

10 

IO 

10 

10 

i A.T. 

IO 

IO 

10 

10 

IO 

10 

30 

15 

IO 

IO 

— 

— 

— 

10 

— 

— 

— 

— 

no 

O 

IO 

w 

150 

no 

no 

no 

no 

no 

14 . 

14 

14 

14 

14 

24 

14 

14 

i -5 

i -5 

9 

i -5 

i -5 

— 

i -5 

i -5 

cover 

cover 

cover 

cover 

cover 

cover 

cover 

cover 






25 

low 1 s high I c 

20 

21 

20 

21 

20 

high 10 

0 

low IO 

910 

1130 

1040 

1400 

1320 

1140 

730 

1400 

1380 

730 

— 

— 

— 

pink 

white 

— 

— 

— 

30.9 

27.7 

3 i -5 

3 i -3 

37-7 

n.9 

29.4 

29 -5 

3.n 

not determined 

2.98 

not deter’d 

5 ° -5 


it 

a 


47.8 

u 

a 

45-9 

36.6 

55-4 

36.3 

4 i -5 

52.9 

43-5 

45-7 



,. j 


_ ,„j 




46:1 

63: 

1 

46: 

1 

3 i: 1 

46 

: 1 


The per cent of copper in the total slag and salt varied from 
. 6 % to 1.15%. 

Fusions 15 and 16, 32 and 33, 35 and 36 show that the 
temperature has very little effect on the amount of copper 
slagged. 






























122 


NOTES ON ASSAYING. 


Borax and soda evidently do not aid it: fusions 39 and 40. 
Fusion 40 shows the effect that the increase of soda has upon 
the size of the lead button. 

Silica seems to help it: fusion 44. A very high ratio of PbO 
to ore or to the copper present in the ore certainly helps the 
slagging of the copper. 

A small-sized lead button, provided it collects the precious 
metals, seems advisable. 

Silver in Antimonial Ores.—On an ore (R.P. = 0) containing 
antimoniate of lead (antimony 25 per cent and lead 40 per cent) 
the following charge gave results fully as satisfactory as the 
scorification method. F crucible was used. 

Ore. i A.T. 

Bicarbonate of» soda. 40 grammes 

Borax glass. 15 “ 

Litharge. 70 “ 

Argols (R.P. = 8). 3 i “ 

Cover of salt. 

Here, as in the case of copper ores, high litharge was used 
in order to oxidize the antimony and either slag it or volatilize it. 

The lead buttons were soft and malleable and cupelled sat¬ 
isfactorily. 

If the litharge is not high, the antimony will go into the lead 
button, which when cupelled will give trouble. If the anti : 
mony is present in large amount, the cupel will be cracked all 
to pieces; if in smaller amount, the edges of the cupel will be 
bulged out, cracked, and a scoria left on the inner surface. 

The following ore (roasted stibnite) will serve as an example. 


Gangue quartz and slate 

; antimony, as 

oxide, i 4 7 /io%. 


A. 


B. 

Ore. 

1 A.T. 

10 

grammes 

Bicarb, of soda. 

30 grammes 

10 

u 

Borax. 

10 

10 

(( 

Litharge. 

60 

90 

(< 

Argols. 



t( 

Silica. 

2 

2 

li 


Cover of salt. Cover of salt. 













ASSAY OF ORES FOR SILVER. 


123 


In A the cupel was coated with scoria and was partly cracked. 
The edges were much bulged and very rough. 

In B the cupel was free from scoria and showed no signs of 
cracking. 

Concentrates, mainly Sb2$3 with some gangue. R.P.=4. 


Concentrates. \ A.T. 

Bicarbonate of soda. 15 grammes 

Borax. 10 

Litharge. 90 

Nitre. 8 

Silica. 3 

Cover of salt. 


< c 


< < 


Pyrrhotite.—This ore or its presence in another, unless it 
is roasted and then assayed, generally gives trouble, for it is a 
difficult one to decompose. All the precautions, previously 
laid down for assaying sulphide ores, should be carefully observed, 
otherwise the final fusion will be unsatisfactory, the slag full 
of lead shot, and the size of the lead button uncertain. 

High soda and a high temperature are necessary, especially 
in finding the reducing-power, otherwise the value will be too 
low, causing subsequent trouble in the regular fusion. The 
following results were obtained upon a pure pyrrhotite: 


Preliminary Fusions. 


Ore, grammes 

3 

2 2 

2 

2 

2 

Bicarb, of soda, 

4 

4 4 

2 

8 

8 

Litharge, 

60 

60 60 

80 

60 

60 


Cover of salt in each case. 




Temperature. 

High 

Low High 

High 

High 

Low 

Lead, grammes. 

18.01 

15.96 17.46 

15.66 

17.81 

17.06 


9 

7.98 8.73 

00 

8.9 

8-53 

Influence of 

' Bicarbonate of Soda 

and Borax. 


Pyrrhotite, grammes. . , 



12 


12 

Bicarb, of soda, “ • . . 


30 

48 


None 

Borax, “ • • < 



None 


20 

Time of fusion, minutes. 


35 

35 


35 

Fusion. 



Very liquid 

* 


* Took a long time, and the result was some slag and a matte (FeS), weighing 


10 grammes. 
















124 


NOTES ON ASSAYING. 


Regular Fusion.—Here the amount of bicarbonate of soda 
must be large whether the iron method is used or some other, 
and the fusion must be a long one. Except in the iron method, 
the litharge must be high, i.e. fully 15 to 20 per cent in excess 
of the amount called for. Borax seems to be of no advantage, 
and little if any need be used except in the fusion with iron. 
Silica is necessary. 


No. of fusion, . . 

1 

2 

3 

4 

5 

6 

7 

8 

Ore, A.T.(R.P.= 

9 ) 2 

h 

I 

h 

h 

h 

i 

i 

Bicarb, soda, grms. 40 

40 

40 

15 

15 

40 

40 

40 

Borax, 

20 

— 

20 

20 

20 

20 

— 

— 

Litharge, 

30 

70 

70 

130 

150 

15 ° 

! 5 ° 

150 

Nitre, 

None 

27 

27 

27 

27 

27 

27 

27 

Silica, 

4 

3 

3 

3 

3 

3 

3 

7 

Nails (20-penny) 

5 

Cover of salt in each case. 




Time, minutes. . 

5 ° 

40 

40 

35 

35 

45 

40 

40 

Matte. 

. None 

Small 

amount 

Small 

amount 

None 

Slight 

coating 

None 

None 

None 

Lead. 

27 

9.8 

12.6 

38 

36 

25 

28 

29 

Slag. 

. Very 

Full of 

Full of 

Full of 

Good 

Clean 

Clean 

Fusion 

liquid 

lead 

lead 

lead 

and 

and 

and 

very fine 


and 

good 

shot 

shot 

shot 

liquid 

liquid liquid and slag 
Crucibles very clean 


little attacked 


Fusions i and 8 were the most satisfactory. 

All the fusions show the necessity of high soda with this ore, 
and fusions No. 2 and 3 show clearly that the litharge cannot 
be cut down, as is the case with many ores, even when the soda 
is high and a large amount of nitre is present. If silica is not 
present in the ore the addition of from 40 to 50 per cent, to 
form an iron silicate, is recommended. 

Ores Carrying Barite.—Fluorspar, as well as fluxes acting as 
acids like silica and borax, is very helpful in decomposing these 
ores. 

The following fusions were made on an ore carrying zinc 
blende and chalcopyrite in a gangue having a high percentage 
.of BaSC>4 in it. The R.P. of the ore was 4. 




ASSAY OF ORES FOR SILVER. 125 


No. of fusion. . . 


1 

2 

3 

4 

5 

h 

6 

7 

Ore, A.T . 


I 


i 

i 

I 

/ 

l 

Bicarb, of soda, 

grammes 

15 

15 



3 ° 

15 

15 

Borax, 

i < 

10 

— 

— 

2 5 


10 

10 

Fluorspar, 

i ( 

— 

15 

3 ° 


15 

5 

— 

Litharge, 

i i 

75 

75 

75 

75 

75 

75 

9 ° 

Nitre, 

( ( 

8 

8 

8 

8 

8 

8 

8 

Silica, 

i ( 

3 — 

Cover of salt in 

each 

case. 

~ 1 

5 

8 

Time of fusion, minutes. .. 

40 

3 ° 

3 ° 

3 ° 

40 

3 ° 

3 ° 

Lead, ] 

grammes. 

26 

27 

— 

— 

27 

20 

24 

Lead and matte, 

< < 

— 

— 

16.3 

7 -2 


_ 


Slag . 


Very 

liquid 

Good 


— A little 
thick 

Very 

fine 

fusion 

Very 
fine 
fusion 
and slag 

Fusions 3 

and 4 

again 

show 

the 

absolute 

necessity 

of the 


presence of soda in the fusion of a sulphide ore. 

Fusions 6 and 7 were the most satisfactory. 

In all the crucible work which has just been described the 
student has not only weighed the ore out accurately, but the 
fluxes, the object being to make him familiar with the chief 
assay reagents, to show him why they are used, why certain 
amounts are taken, and what influence the reagents have upon 
each other. If he understands their action and the theory of 
their use, then he should be able to assay any ore. If he does 
not understand them, then it becomes a mere matter of guess¬ 
work. 

Now in the regular work of a busy laboratory it is impossible 
to weigh out each reagent, so there is always a general flux mix¬ 
ture kept on hand and a certain measured amount is taken, pro¬ 
portional to the weight of ore used. The ore of course is 
weighed out accurately, the general flux taken by measure and 
anything else added, which, from its character, the assayer judges 
the ore requires. 

One flux mixture has been given on page 113. Another, used 
in a Western laboratory, consists of: 

2.07 kilos, of KoCO,, 

■ »Ntfo„ 

2.55 “ “ borax glass, 

.45 “ “ flour, 

13.6 “ “ litharge. 






126 


NOTES ON ASSAYING. 



Another mixture consists of 

9 kilos, of litharge, 

9 “ “ borax, 

n to 13 “ “ bicarbonate of soda, 

113 grammes charcoal. 

It will be noticed in these mixtures that flour is used as a 
reducing agent in place of argols, which are given in these notes. 
One may not be able to obtain argols, but one can always obtain 
either charcoal, flour, starch, or something which will act as a 
reducing agent and answer just as well to throw down a lead 
button. Any one studying assaying in a well-equipped labora¬ 
tory is expected to learn the reasons for the different steps in his 
work; after leaving one must adapt himself to his surroundings 
and the conditions he finds, which he can easily do if he has a 
good fundamental knowledge. If he lacks this knowledge and 
simply works by rule of thumb and uses a flux mixture which 
he has found to work successfully on some simple ore from a cer¬ 
tain district, he will surely be in trouble when he meets with some 
difficult ore from elsewhere. 

All silver in an ore above one ounce is paid for, the price 
being 95% of the New York quotation for silver at the time of 
sale of the ore. 


CHAPTER IV. 


ASSAY OF ORES FOR GOLD. 

Gold fuses at 1064° C. Sp. gr. = 19.3. Atomic weight = 197.2. 

The first thing one should bear in mind in assaying ores for 
gold is that, even in a fair-grade ore, we are working upon material 
which is carrying an extremely low percentage of the metal. An 
ore carrying J oz. of gold per ton of 2000 lbs. av., or .0005 gramme 
to the A.T., is a good ore, and many ores carrying only .2 oz. 
($4_ ul ) to the ton, or .0002 grammes to the A.T. are worked at a 
profit. 

With such a small amount to weigh and base our results 
upon, the student can easily see: 

1 st. That he must be perfectly exact in all his work. 

2d. That, as a rule, the amount of ore taken should be larger 
than we use in the assay of ores for silver or lead. 

3d. That, in order to obtain satisfactory results, it is necessary 
to have the sample pulverized extremely fine. That is, the ore 
should pass a 120-mesh sieve at least, and in many cases a 160- 
mesh or 200-mesh sieve is none too fine. 

Any of the ores the student has previously assayed for silver 
may also contain gold. 

As a rule, gold occurs chiefly in veins of quartz, but it is also 
found in slate, granite, gneiss, and syenite. 

One saying is, “gold, is found wherever you find it,” and 
Cripple Creek seems to illustrate this. It occurs both native 
and associated with sulphides. Pyrite and arsenopyrite are 
most frequently met with, but any sulphides, such as galena, 
chalcopyrite, and blende, may be auriferous. Above the water 
level most veins are heavily stained with iron oxide, due to the 

127 


128 


NOTES ON ASSAYING. 


weathering of pyrite, arsenopyrite, or any sulphide carrying iron. 
The following are very rich gold-bearing minerals: 

! Te 60% approximately. 

Au 30% “ 

. Agio% 

C Te 58% 

Calaverite (telluride of gold), sp. gr. 9.04.-s Au 39% “ 

<Ag 3% 

Petzite (telluride of silver and gold), sp. gr. =8.7 to 9.2 

Ag 40% to 50%; Au 24% to 25%, the remainder tellurium. 

Foliated tellurium carries Au, Pb, Te, S, and Sb. 

In making the assay we have the following steps: 

1st. Collection of the gold and silver in the ore by means of 
lead in the scorification process, and by means of litharge, reduced 
to lead, in the crucible process. The gangue and the impurities 
in the ore pass into the slag. 

2d. Cupellation of the resulting lead button. 

3d. Weighing the Au+Ag bead, if the Ag is to be determined. 
4th. Inquartation of the bead if found necessary. 

5th. Parting the button in HN 0 3 or H 2 S 0 4 , washing with 
H 2 0 , and transferring the gold to an annealing-cup or por¬ 
celain crucible. 

6th. Drying, heating, and weighing the gold. 

Gold ores may be divided into: 

Class I. Ores with no sulphides, arsenides, or material of a 
reducing nature in the gangue. 

Class II. Ores with sulphides, arsenides, etc., in the gangue, 
or ores with a reducing power. 

Class III. Telluride ores. 

METHODS THAT MAY BE USED FOR ASSAYING. 

Class I. (Scorification Method.) A. Do not use this method 
unless it is necessary as, for instance , in the case of copper ingots 
or bars and material rich in copper , zinc residues from the KCy 
process and similar material not suitable for the crucible assay. 
The loss of gold is greater in scorifying than in cupelling. If 
obliged to use this method, take A.T. of the substance and 
proceed as described under the Assay of Copper Matte and Zinc 


ASSAY OF ORES FOR GOLD. 


129 


Residues for Gold. If the ore is poor, it may be necessary to 
take six or more portions. Finally combine these and base the 
result in gold upon the total substance taken. 

Some assayers use 4" to 5" scorifiers, take \ A.T. of ore, a 
large amount of PbO, borax glass and some reducing agent, and 
fuse. This is not strictly a scorification, but rather a fusion or 
melt in a scorifier, and there is often difficulty in obtaining a 
lead button of the proper size. 

Class I. (Crucible Method.) B. This may be done either 
in a pot-furnace or in a muffle. It has the great advantage 
over the scorification in that a much larger quantity of ore can 
be used, i.e., 1 A.T. or more. 

As in the assay of ores for silver, the active fluxes are two 
or more times the amount of the ore taken. 

Use G or H crucibles and fuse in a pot-furnace, as described 
in the assay of ores for silver. 

The following will serve as illustrations of charges to be used 
on ores of different character. 


No. 1. 
Silicious 
Ore. 



Ore, A.T. 

i 

(a) 

1 

(&) 

1 

V 

G (D 

Bicarb, soda, 

gms. 

60 

30 

sa. 

Borax, 

<k 

5 

S 

* §1 

Litharge, 

(4 

3 ° 

60 

S 0 

Argols (R.P.= 

10), “ 

2* 

2* 


^ Silica (Si 0 2 ), “ — 

cover of salt 

Lead button, gms. — — 

No. 6. 

Roasted Ore 
containing 
12% Copper. 



f Ore. 

. * A.T. 

0 

.£ O 

Bicarb, soda, 

gms. 1S 


Borax, 

10 

.3 0 
* 2 

PbO, 

“ 90-110 

3 o1 

Argols (R.P. = 

. Si 0 2 , 

10), gm. 2{ 

“ 3 


Cover of salt 

A large amount of argols cannot 
be used, for copper will be reduced, 
therefore it is always advisable to 
determine the oxidizing power of 
the ore so as to obtain about a 20- 
gramme lead button. 

* Or sufficient to reduce the Pb. (See 


No. 2. 

No. 3. 

No. 

4. No. 

5 - 

imestone. Ore contain- 

- Hematite. Roasted 


ing Fe 2 0 3 


Concentrates. 


either 


Oxidizing 


native or 


power = 

= t- 


due to 




roasting FeS 2 

• 

(a) 

( b ) 

1 

1 

1 

i 

i 

40 

40 

30 

30 

20 

15-25 

20 bor. gl. 8 

borax 10 

10 

30 

30 

70 

30 

100 

2} 

7 * 

3 

2} 

4 

5 

4 

7 + 

3 

7 + 

cover of salt cover 

of salt 

cover of salt 

— 

— 

25 

16$- 

30 




No. 7. 




Ore carrying 43% Cr 2 0 3 . 



(a) 

(b) 

(c) 

I 

f Ore. . . 


1 A.T. 

1 A.T. 1 

A.T. 

1 

! Bicarb. 

soda, gm. 

60 

60 

30 

s! 

Borax, 

44 

20 

— 

— 


Litharge, 

35 

35 

70 


j Argols, 

ii 

3 

3 

3 


[ Glass, 

44 

15 : 

Si 0 2 15 Glass 

20 


Cover of salt in each case 

Time, minutes . .. 

. 45 

35 

35 

Lead, grammes. . . 

. 26 

29 

— 


7 a and 76 fused well. 

7 c gave a slag full of lead shot; slag too basic. 
Slags and crucibles yellow, due to chromates 
of lead and soda, 
s 73, 89, and 90.) 


t See pages 75 and 76. 








I 3° 


NOTES ON ASSAYING. 


If more than i A.T. of ore is taken, increase the fluxes some¬ 
what. Do not change the litharge where 30 grammes are used, 
and take only sufficient reducing agent to give a lead button of 
the desired size. Personally I do not believe in using more than 
1 A.T. of ore in a G or H crucible because sufficient fluxes can¬ 
not be added to decompose the ore without the charge boiling 
over, yet I know that some assayers use as high as 4 A.T. 

If an ore contains copper running not over 25%, assay it for 
gold as described in the Assay of Ores for Silver, Special 
Methods. That is, take only J A.T. of ore in place of 1 A.T., 
use high litharge and silica in order to slag the copper, and an 
amount of reducing agent sufficient to just bring down a lead 
button of the desired size. Keep the active fluxes two or more 
times the ore, as described under assay of ores for silver. 

Fusion in the Muffle.—This is conducted as described under 
the Assay of Ores for Silver. Use an A or B crucible. 

The charge is made up on the following lines: 


Ore. 

1 

A.T. 

Bicarb, soda. 

. 15- 10 

grammes 

Borax glass. 

. o -5 

u 

Litharge. 


u 

Argols (R.P. = 10). 

. 4 

u 

Glass. 


({ 

or 



Silica. 

. i -3 

(i 


Cover of salt. 


If an ore is so poor or low grade that an unweighable amount 
of gold is obtained from 1 A.T., then I think it advisable to pro¬ 
ceed cs follows: 

Very Low Grade Ores or Tailings.—Ores of this character 
can be assayed by either of the following methods: 

1. Take 5, 10, or more portions of 1 A.T. each, fuse in G 
or H crucibles, add silver to each lead button, cupel separately, 
and part all the silver and gold beads in one flask. 

2. Take 5 A.T. of ore and fuse in a K or L crucible witji 
the following fluxes: 









ASSAY OF ORES FOR o Ulu. 


1 3 l 


Bicarbonate of soda. 150 grammes 

Borax. 20 to 25 “ 

Litharge. 300 “ 

Glass. 20 to 25 “ 

Argols (R.P. =9). 10 to 12 “ 

Cover of salt. 


The actual time of fusion is 35 to 45 minutes, but owing 
to the size of the crucible and the amount of the charge, it will 
take 1 hour to ij hours from the time the crucible is placed in 
the furnace until the charge is poured. 

The button of lead (90 to 120 grammes) may be scorified 
once in a 3J" scorifier by pouring off the slag, and the resulting 
lead button cupelled with silver, or it may be cut into four pieces, 
silver added to each and cupelled. The four silver and gold 
beads are parted in one flask. When a small amount of gold 
is present, the loss of gold is practically the same in the two pro¬ 
cedures. 

Method No. 1 gave on one lot of tailings .026 ounces per ton. 

“ No. 2 “ “ same “ “ “ .024 “ “ “ 


Class II. (Crucible Method.) (Ores with sulphides, arsen¬ 
ides, organic matter, or material of any kind in the gangue, with 
a reducing power.) Use G or H crucibles. 

Method C. —Scorify and treat as in Class I, A—method espe¬ 
cially adapted to copper mattes. 

Method D. —Roast the ore and then treat as in examples 3, 5, 
or 6, Class I, B. 

Method E. (Method with Iron. G or H crucible.)—This 
requires iron in excess, a large amount of soda, and a fixed 
amount oj PbO. It is not recommended jor ores containing copper 
because the litharge used is small; nor jor arsenic and antimony 
compounds , jor a speiss may be obtained unless special precautions 
are taken 







132 


NOTES ON ASSAYING. 


Ore. i A.T. 

Soda. 60 grammes, always twice the ore . 

Borax. 5-20 “ 

Litharge. 30 “ 

Argols ) 

Iron > See more complete description of the process. 

Si 0 2 ) 

Method F. (High litharge.)—This requires a large quantity 
of litharge and a small quantity of soda and nitre if necessary. 
A preliminary fusion must be made to determine the reducing 
power of the ore. 

The method is especially adapted to ores containing arsenic, 
antimony, or large quantities of FeS 2 , also to copper ores, when 
carrying up to 25 or 30 per cent of that metal. Never use iron 
in this assay. 

Method G. (Fusion in the muffle.)—Use \ A.T. of ore in 
an A or B crucible. The PbO is high, the soda is low, the borax 
glass considerable, and nitre is used if found necessary. The 
object is to have a fusible slag and a charge that will not boil up 
much in the crucible. 

The following description will give a more complete account 
of the foregoing methods. 

Class II, C.—If obliged to use this method, see Assay of 
Copper Matte and Zinc Residues for Gold. 

Class II, D. (Certain ores may lose considerable gold 
under this method of procedure.)—The method of roasting ores 
containing sulphur, arsenic, or organic matter is carried out 
as follows: 

Take 1 A.T. or more of ore, place it in a clay or iron dish 
made for roasting purposes, and heat the ore very gradually in 
a muffle. The more heavily sulphuretted the ore is and the 
more fusible the sulphides there are in it, such as PbS, Sb 2 S 3 , 
and Ag 2 S, the more carefully the roasting should be performed. 
Stir the ore constantly at first, to prevent its caking. Ij it does 
cake, throw it away and commence again. 






ASSAY OF ORES FOR GOLD. 


*33 


The reaction with FeS 2 is as follows: 

2FeS 2 + n 0 = Fe 2 0 3 +4S0 2 . 

That is, we desire to convert the sulphides in the ore into oxides 
and volatilize the S, As, and Sb. Some sulphates and arseniates 
will also be formed. Some of these can be broken up by heat 
alone (2FeS0 4 = Fe 2 0 3 +S 0 2 -f-S 0 3 or FeS 0 4 = FeO+S 0 3 and 
2 CuS 0 4 = Cu 2 0 +O + 2SO a or CuS 0 4 =CuO + S 0 2 +O). S 0 3 

breaks up into S 0 2 and O. By the addition of carbon we can 
form C 0 2 , S 0 2 , sulphide of carbon and the sulphide of the metal, 
which will then break up and decompose. (2CuS0 4 + 3C = Cu 2 S 
+ S0 2 +3C0 2 .) Carbonate of ammonia may also be used, 
when sulphate of ammonia is formed, which immediately vola¬ 
tilizes. Towards the end of the roast, increase the heat to almost 
scorifying temperature, or add charcoal until no S 0 2 can be 
detected. If charcoal is to be used, remove the dish from the 
muffle, allow the ore to cool slightly, then add the charcoal and 
stir it in well. It is next to impossible to dead-roast an anti- 
monial or arsenical ore, and if arsenic is present, the odor will be 
noticed at this time. 

Everything in the ore is now in the condition of oxide, except 
the gold (which is in a metallic condition), and some lime and 
lead, which, if present in the ore, remain as sulphates. Unless 
the roasted ore contains copper or other metals easily reduced, 
assay as in examples 3, 5, or 6 , Class I, B. 

Class II, E. (Iron Method. Pot-furnace. See also page 
109.)—The great advantage of this method is that it saves making 
a preliminary fusion and finding the reducing power of the ore. 
The student, by this time, ought to be able to judge fairly well 
in regard to the ore, its composition, how to make up a charge, 
and what amount, if any, of argols, iron, and silica is necessary. For 
instance, suppose we take any ore to be assayed and van it. 
What we obtain on the vanning-shovel will depend upon the 
character of the ore. If we obtain a “fan” like that shown in 
the cut and no arsenical or cupriferous minerals are present, 
the charge will be made up as follows: 


*34 


NOTES ON ASSAYING. 


u 


u 


u 


Ore. i A.T. 

Bicarb, soda,-60 grammes 

Borax. 5 

Litharge.40 

Argols(R.P. = 10) 2 

Cover of salt. 
The argols will reduce 20 
grammes of lead, and we rely on the ore to give us 5 to 8 grammes 
more lead, still leaving some PbO in excess. No iron or silica 
is needed. 

If the ore, upon vanning, shows a 
“fan” as in the figure annexed, the charge / FeS a 

ZnS 



can be 

made up 

in either of 

two ways: 

Ore. 

. . . 1 A.T. 

Ore... .<»?. 

.... 1 A.T. 

Bicarb.soda. 60 gm. 

Bicarb, soda 

. ... 30 gm. 

Borax... 

... 10 “ 

Borax. 


PbO. ... 

... 30 “ 

PbO. 

....60 “ 

Argols. . 


Argols. 


Iron. . . . , 


Iron. 


Si0 2 - 

. . . none 

SiOo . 




a 


u 


Cover of salt Cover of salt 

In a the ore is decomposed by both the iron and the litharge 
and we have a lead button of the desired size. In b the PbO 
decomposes the ore and in so doing gives the lead button. 

If now the ore, upon vanning, shows a “fan” similar to the 
figure below, the charge is made up as follows: 

Ore. 1 A.T. 

Bicarb, soda. 60 grammes 

Borax.10-20 

PbO. 30 

Iron nails (twentypenny). 4 

Si 0 2 . 3-5 grammes 

Cover of salt. 

Reactions in last Fusion: 

FeS 2 + 5PbO = FeO + 2S0 2 + 5Pb; PbS + Fe = FeS + Pb; 

PbS + 2 PbO = S 0 2 + 3 Pb; PbO + Fe = FeO + Pb; 

FeS 2 + Fe = 2FeS; FeO + Si 0 2 = FeO,Si 0 2 . 

Some iron oxide is formed by the oxidation and corrosion 
of the iron nails just at the surface of the fusion. 
































ASSAY OF ORES FOR GOLD. 


*35 


The FeO goes into the slag or combines with the Si 0 2 from 
the crucible or with that added. 

The FeS dissolves in the highly alkaline slag, and the S 0 2 
goes off as a gas or oxidizes to SO s and forms Na 2 S 0 4 . 

In the last example it is very evident that 30 grammes of 
litharge will not decompose the ore taken, so a large amount of 
iron is necessary and the Si 0 2 is added to combine with the 
FeO formed during the fusion, which would otherwise com¬ 
bine with the Si 0 2 of the crucible itself. 

The amount of Si 0 2 added depends upon the amount of 
sulphides present in the sample. The larger the amount of 
sulphides the smaller must be the gangue and hence the amount 
of Si 0 2 added, to combine with the FeO and other oxides formed, 
must be large. 

This method (E) has the following advantages: 

1st. A lead button of the proper size can be obtained, for 
the quantity of litharge used is small. 

2d. The crucible charge varies only slightly with different 
ores. 

The disadvantages are: 

1st. The difficulty of both removing the iron or nails from 
the fusion and of having them free from lead globules. 

2d. The tendency of the slag to be rather thick, basic, and 
corrosive. 

As previously stated, for 1 A.T. of ore, the soda is 60 grammes 
and the PbO 30 grammes, unless the ore carries lead when less 
than 30 FbO is used. The borax and Si 0 2 must be increased, as 
the sulphides in the ore increase and as the gangue becomes basic. 

All the gold must be extracted from the ore, and in order to 
do this all sulphides present in the ore must be decomposed. 

Fusion. (Gold Ores, Crucible Method, with the Use of Iron.)— 
The fluxes are first weighed into the crucible , and the ore last 0j 
all. Mix thoroughly, hit the crucible on the outside, put the 
nails in point down, and then cover with £" of salt. Conduct 
the assay exactly as described in the assay of a silver ore by 
crucible method (pages 83 and 90). After the ore has been fused 
20 minutes lift out the nails and see if they are cutting or eating 
off at the surface of the slag; if so, add one or two fresh ones, 


i 3 6 


NOTES ON ASSAYING. 


leaving the others in the crucible. If the nails cut off, it is 
not only difficult to remove them, but it renders a satisfactory 
pour impossible. The iron present in the slag should be in the 
condition of ferrous oxide—ferric oxide tends to retain gold in 
the slag. Fuse for 40 to 45 minutes or until no drops of lead 
are seen adhering to the nails, when they are raised out of the 
fusion. When the fusion is completed, remove them, and holding 
them partly in the fusion, tap them gently to knock off any adher¬ 
ing drops of metal. Let crucible stay in the furnace two or three 
minutes longer, then pour. The lead button is cupelled as usual, 
but it may not start to drive quite as quickly as other buttons, 
owing to a film of oxide iron that is present. 

Arsenical Ores.— Students are advised not to use the iron 
method on arsenical ores until they have had considerable experi¬ 
ence , for special precautions are necessary. 

The following experiments by Mr. G. Barnaby, 1904, will 
be found of interest in this connection. 

Arsenopyrite is a very common mineral in Nova Scotia gold 
ores, and when the tailings from stamp-mills are concentrated 
the concentrates often carry a high percentage of arsenic, 30 
and 40 per cent being not uncommon. The following is the 
partial analysis of gold ore No. 1462 A. 

Silica. 31-5°% 

Fe. 20.61% 

Arsenic. 20- 53 % 

Sulphur. 11.04% 

Reducing Power, 3.6 

When no iron was used in the fusion the following results 
were obtained: 


Weight of ore. 

.. A A.T. 

A a.t. 

i A.T. 

A a.t. 

Bicarb, soda, grammes. 


10 

25 

20 

Borax, “ . 


— 

— 

— 

PbO, “ . 


90 

70 

60 


Nitre, 

Silica, “ 

Iron, 

Time fusion, minutes. . . 
Weight of Pb, grammes 
“ “ Au, 

Ounces per ton. 


1 

2 


Cover of salt in each fusion. 

45 27 

, 20.2 25.6 

. .00084 .00084 

. 2.8 2.8 


45 

32.3 

00140 

2.8 


45 

30 

.00084 

2.8 

















14^3 A. FUSIONS IN THE POT-FURNACE WITH IRON AND EXCESS OF ALKALI. (See also page 7 *-) 


ASSAY OF ORES FOR GOLD. 


137 


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138 


NOTES ON ASSAYING. 


A second sample, 1462 D, consisting of concentrates with 
a R.P. of 7.4 and carrying 36.2 per cent arsenic gave the follow¬ 
ing results. Ore through 100 sieve. Assay 2.47 oz. gold. 


No. of Fusion. 

Ore used. 

Sodium bicarb., gms. .. 
Borax, “ . .. 

Litharge, “ . .. 

Silica, “ . .. 

Iron nails(2openny), gm. 

Salt. 

Time of fusion, minutes 

Temperature, deg. j 

Lead, gms. 

Speiss, “. 

Iron consumed, “. 

Gold in lead, “. 

Ounces per ton. •. 


No. of Fusion... 

Ore used . 

Sodium bicarb., gms 
Borax, 

Litharge, “ 

Silica, “ 

Iron nails(2openny), gm. 

Salt. 

Time of fusion, minutes 


Temperature, deg. C. j 

Lead, gms. 

Speiss “ . ... 

Iron consumed, “ . ... 
Gold in lead, “ . ..., 
Ounces per ton. 


L. 

H, 


40 

.3 A.T. 
30 
5 

30 

3 

70.7 

cover 

45 

usual 
1280 
26. 7 

13-33 
22.6 
.00067 
2.23 


42 

.3 A.T. 
3 ° 

5 

30 

3 

54-2 

cover 

45 

25'725 

20' 120c 

26.7 

7-83 

19.7 
.00071 
2-37 


00 

’'t 

41 

45 

49 

.3 A.T. .3 A.T. .3 A.T. 

.3 A.T. 

.3 A.T* 

60 120 

3 ° 

60 

120 

5 5 

5 

5 

5 

3 ° 3 ° 

30 

30 

30 

3 3 

3 

3 

3 

7 o -9 53-8 

51.8 

52.1 

5 o -3 

cover cover cover 

cover 

cover 

45 45 

45 

45 

45 

usual usual 

low 

low 

low 

1235 1185 

710 

700 

575 C. 

27.2 26.3 

26.8 

27-5 

24.8 

11.44 6.42 3.61 

1.04 

0 

24 29.6 

10 

8.2 

7 -i 

.00069 .00070 .00074 

.00072 

j not deter- 

2-3 2.33 2.47 

2.40' 

j mined 

46 

43 

47 


.3 A.T. 

.3 A.T. 

.3 A.T. 

.3 A.T. 

60 

30 

60 

30 

5 

5 

5 

5 

30 

30 

60 

30 

3 

3 

3 

3 

53-4 

53-5 

55 

‘ 52 

cover 

cover 

cover 

cover 

45 

45 

45 

10 

L. 25' 735 H. 
H. 20' 1235 L. 

25' 1370 H. 25' 1400 
20' L. 20' 

usual 

26.9 

26.5 

26.7 

not 

weighed 

7.24 

14.58 

11.92 

3-3 

26.7 

25.2 

29.2 

— 

.00069 

.00059 

not determined 

2 -3 

1.96 

< < 

i c 


The conclusions that can be drawn from the results on these 

two samples seem to be: 

1 st. That a high temperature is conducive to the formation 
of a speiss. 

2d. That the size of the speiss obtained in a fusion depends 


















ASSAY OF ORES FOR GOLD. 


T 39 


upon the temperature and the amount of alkali (soda or potash) 
used in the charge. 

3d. That at a high temperature the speiss begins to form as 
goon as the charge fuses (fusions 26 and 28). 

4th. That the speiss may or may not carry gold. 

5th. That a large speiss button usually carries gold and the 
results from the lead alone are consequently low. 

6th. Both too high and too low a temperature should be 
avoided. The latter (fusions n, 12, and 13) give low results, 
owing to incomplete decomposition of the ore. 

7th. That the best temperature, at first, is one as low as the 
fusion can be conducted and yet have the ore decomposed, finish¬ 
ing with high (fusions 14, 15, 16, and 35). Fusions made in this 
way give either no speiss or so small a one that the gold contents 
will not be appreciable. 

8th. That as the alkali flux increases the iron consumed at a 
given temperature diminishes, due no doubt to the formation of 
arseniate and arsenite of soda, both of which are found in the slag,, 
rather than to the formation of arsenide of iron. 

If ores, carrying as high a percentage of arsenic as these, can 
be assayed by this method and no speiss result, it would seem as 
though the method could be satisfactorily used where only a 
small percentage of arsenic was present, simply by increasing 
the alkaline flux and maintaining the correct temperature. 

Iron Method. Fusion in the Muffle.—Ores in this class may 
be fused in the muffle also, as described under the Assay of Silver 
Ores, page 92, using \ A.T. of ore. 

Class II, F. Large excess of PbO and nitre if found neces¬ 
sary. (See also Class II, Silver Ores, page 93.)—In taking up 
this method first refer to and consider what took place when we 
found the reducing power of charcoal and argols (page 83), and 
also what took place in determining the reducing power of silver 
ores, page 94 to 102. Every ore containing sulphides, arsenides,, 
etc., or with a reducing power, should have this reducing power 
determined in the same manner by a trial or preliminary fusion. 

Proceed as in Class II, pages 103 to 106. buse for 10 to 20 
minutes or until fusion is quiet. Pour and weigh the resulting 


140 


NOTES ON ASSAYING. 


lead button and calculate the reducing power of the ore. Sup¬ 
pose 2 grammes of the ore gave 14.8 grammes of lead, then the 
R.P. = 7.4. 

Keeping in mind that litharge (PbO) and nitre (KN 0 3 ) are 
both strong oxidizing agents and that they are both able to decom¬ 
pose sulphides and similar compounds, it follows that, having 
found the working reducing power of any ore, we can make up a 
charge, as we did in the assay of ores for silver, to obtain a result¬ 
ing lead button of any size we wish. This lead button should 
carry all the silver and gold in the ore. Ores to be treated by 
this method may be divided into three subclasses, based upon the 
quantity of reducing material, i.e., sulphides, arsenides, etc., 
present in the sample. 

a. Ores requiring a reducing agent in the regular fusion. 

b. “ “ no “ “ “ “ 

c. “ “ an oxidizing “ “ “ 

1 A.T. of ore is used in the regular fusion unless it contains 
much copper or has a reducing power of 4J or over, when only 
\ A.T. is taken. 

The following will serve as examples: 


No. 10. No. 26. No. 3c. 

Suppose the preliminary fusion on.. io 5 3 

gave a lead button of. 3 5 12 

Then the working R.P. =. .31 4 


No. 4 c. 

2 grammes ore 
14.S grammes. 

7-4 


Make up the charges for the regular fusion as follows, using 
G or H crucibles in pot-furnace: 


No. 1 a. 


Ore. 1 A.T. 

Sodium bicarb, grms. 30 

Borax, “ .... 5 -I ° 

Litharge, “ .... 60-75 

Argols (R.P. = 9), “ .... 2 

Nitre (O.P. = 4.2), “ 


No. 26. 

No. 3c. 

No. 4 c. 


x y 


i A.T. 

1 A.T. or 1 A.T. 

£ A.T 

3 ° 

30 60 

15 

5-10 

10 10 

10 

75 

140 90 

130 

— 

21 21 

19 

5 

25 — 

20 


Glass, 
or SiO z , 

Cover of salt in each case. 







ASSAY OF ORES FOR GOLD. 141 

We aim to obtain a lead button weighing between 26 and 30 
grammes. 


No. 1 a. 

We require some reduc¬ 
ing agent in this fusion, 
because 1 A.T. of ore 
will reduce only 8.7 
grammes of lead. 
Therefore we add 
enough argols to re¬ 
duce 18 more grammes 
of lead. 


No. 2 b. 

There is sufficient reduc¬ 
ing material in this 
ore to give a 29-gm. 
lead button. There¬ 
fore we require neither 
argols (reducing agent) 
nor nitre (oxidizing 
agent). 


No. 3c. 

In charge x the ore will 
reduce 29.166 X 4 = 
116.6 grammes of 
lead. We desire a 
26-gramme button. 
.’. 116.6 — 26 = 90.6 
grammes of lead to 
be oxidized. 

Oxidizing power of nitre 
90.6 

= 4-2 .-= 21 gm. 

4.2 

of nitre to be added. 


No. 3c. In charge y, the 30 additional grammes of soda 
take the place of the 50 grammes of PbO left off. Owing to 
this diminution of litharge no silica is used, otherwise the charge 
is the same as x. 

No. 4 c. Here \ A.T. of ore is used, but the charge is made 
up on the same basis as No. 3 c X] the amount of litharge in each 
case is 20 per cent in excess of the total amount of lead that each 
ore will reduce, i.e., ore No. 3c x will reduce 29.16X4=116.6 
grammes; 20 per cent of this is 23, or a total of 140 grammes. 

This is lead, but it is sufficiently close to call it litharge with¬ 
out figuring the exact amount of litharge that will yield 140 
grammes of lead. 

Summing up this method, the first thing necessary is to 
determine the working reducing power oj the ore and then the 
oxidizing power oj the nitre y after which calculate the total 
amount of lead that the ore intended to be used in the regular 
fusion will reduce; if less than 30 grammes, add sufficient 
argols to make up the difference; if it will reduce more than 
30 grammes, subtract 25 to 30 from the amount reduced, not from 
the PbO used, and divide the difference by the oxidizing power of 
the nitre; this will give the amount of nitre necessary to add to 
the regular fusion. 

Regular Fusion.—The fusion is made in the pot-furnace in 
the usual manner. When the charge begins to fuse, check the 


142 


NOTES ON ASSAYING. 


fire at once, to have the charge fuse quietly. The more nitre 
there is present the greater the care to be observed , because the action 
at times is very violent. The nitre and PbO both decompose 
the sulphides present, and the nitre no doubt oxidizes some of 
the lead reduced, when it is in small globules, but in what order, 
if any, these reactions take place it is difficult to say. 

Most ores will be decomposed by a fusion of 25 to 30 minutes, 
but for heavily sulphuretted ores 50 minutes is sometimes neces¬ 
sary. 

The disadvantages of the process are: 

1 st. The necessity for a preliminary fusion. 

2d. The liability of an excess of PbO eating through the 
crucibles. 

3d. The possibility of obtaining a button differing in weight 
from that figured for. 

The last two can be avoided, if due care is given to the work 
and ij the same ratio oj soda to ore is maintained in the regular 
jusion as was used in the preliminary one. An excess of soda 
(as shown under Silver Assay, page 97 and following) seems 
to tend to form S 0 3 rather than S 0 2 , and consequently the amount 
of lead obtained is greater when a large excess of alkali is used. 
This method is especially adapted to arsenical , antimonial , 
and copper ores. The first two are oxidized and either volatilized 
or slagged, and the last is slagged by the excess of litharge used. 
The lead button, if soft and malleable, is cupelled as usual. 

Class II, F. (Fusion in the Muffle.)—Ores in this class can 
be fused in the muffle, as described under the Assay of Ores for 
Silver. For instance, the charge for ore 3c on page 140 would 
be made up as follows. Use a B crucible. 


Ore. 

Sodium bicarbonate 

Borax glass. 

Litharge. 

Nitre (O.P. =4.2).. 
Glass. 


i A.T. 

. 10 grammes 

. 10 “ 

, 90 “ 

7 “ 

■ 5 

Cover of salt. 









ASSAY OF ORES FOR GOLD . 


143 


The fusion would have to be made very carefully owing to 
the nitre present. 

Fuse 40 to 50 minutes. 

Class III. Telluride Ores.—Ores carrying tellurium com¬ 
pounds are certainly more difficult to assay for gold than the 
•ordinary run of ores, and when there is a large percentage of 
tellurium present satisfactory results are extremely difficult to 
■obtain. At one time the scorification method was supposed 
to be the only satisfactory one, but equally uniform results can 
be obtained by the crucible method, if all due precautions are 
taken and the ore is sufficiently fine (170-to 200-mesh). In the 
former a large amount of lead and a high temperature are essen¬ 
tial, and in the latter a high temperature and a large amount of 
alkali. The idea is to have the tellurium form a tellurate of 
soda and enter the slag or else be oxidized by the litharge. Some¬ 
times one button, among several assays conducted under con¬ 
ditions as near alike as possible, carries much more tellurium 
than the others, and results on some rich ores lead me to believe 
that, in these instances, the tellurium compounds which have 
entered the slag are broken up and the tellurium enters the lead 
button. Much gold can be lost when roasting ores rich in both 
gold and tellurium. Low-grade ores, when little tellurium is 
present, can be roasted with little loss of gold, and assays on these 
roasted ores will always run much more uniformly than upon the 
raw ores. It will be seen from these preliminary remarks that 
there is room for much valuable investigation upon this class of 
ores. 

Numerous papers have been published in regard to the cor¬ 
rect method of assaying these ores, and many different charges 
and methods have been suggested. No doubt they all have 
their advocates, and in their hands satisfactory results may be 
•obtained. The thing more important than all others is to see 
that the ore has been ground to a sufficient degree of fineness. 
For the majority of rich ores this is not less than what will pass 
through a 170-mesh sieve, or finer if possible. In other words, 
the richer the ore the finer the sieve should be through which 
it should be passed. 

The following experiments were carried out by Mr. A. L. 


144 


NOTES ON ASSAYING. 


Davis of the class of 1898, and were first published in the Tech¬ 
nology Quarterly , Vol. XII, No. 2, June 1899: 

“The ore selected for the work came from Boulder County, 
Colorado, and gave a strong test for tellurium. The per- 
centage was not determined. The gangue was chiefly quartz, 
with tellurides and pyrite scattered through it. It was pul¬ 
verized and passed through a 100-mesh sieve. Assays made 
by myself showed 68.6 ounces gold by the scorification method, 
and 68.3 ounces gold by the crucible method of assay, no cor¬ 
rection being made for gold left in slag and cupel. The ore 
had a reducing power of 1.2. 

“ In the experiments carried out by Mr. Davis four different 
crucible charges were tried, and three different scorification 
charges. In each experiment the slags and cupels were saved, 
ground separately and assayed, in order to trace the loss of gold, 
if there was any. The results are shown in the accompanying 
tables. From the tables it will be seen that, in the scorification 
process, some gold is always lost in the slag, and that it is larger 
than the loss sustained in cupellation, which of course is the 
rule in gold assays. In the crucible process there is also a loss 


SCORIFICATION METHOD. 


Charges. 

J No. of Experiment. 

Weight of Lead Button 

Wt. after Rescorifying. 

Wt. after 3d Scorifica¬ 
tion. 

Wt. of Gold after Part¬ 
ing (Grammes). 

Gold Found in Slag. 

'3 

a 

n 

0 

c 

• tH 

0 

Pa 

O 

O 

Total Gold. 

Ounces per Ton. 

Per Cent of Total Gold 
Found in Slag. 

Per Cent of Total Gold 
Found in Cupel. 


Ore j\; A.T. .. 

ia 

30 

20 - 

.00622 

.00005 

.00004 

.00631 

63.1 

• 79 

. 63 

T J 

Gran, lead, 60 gms. 

2 b 

30 

cupelled 

.00648 

— 

— 

— 

64.8 

_L 



(£ mixed with ore, 












i placed on top). 

3 C 

3 i 

cupelled 

.00690 

trace 

trace 

.00690 

69 

. 00 

. OO 

Borax glass, a pinch on 











top 

of all. 












Ore rV A.T. 

le 

34 

22 - 

.00686 

.00004 

none 

.00690 

69 

• 58 

none 


Gran, lead, 45 gms. 











II. -1 

(£ mixed with 












ore). 

2 f 

25 

cupelled 

.00648 

.00005 

.00005 

.00658 

65.8 

■ 76 

. 76 


PbO cover 10 gms. 

3 g 

25 

cupelled 

.00650 

.00010 

.00006 

.00666 

66.6 

I . 50 

. OO 

Borax 

glass, 0.3 gms. on 











top of all. 











\ Ore A.T . 

i h 

48 

37 20 

. 00684 

. 00005 

none 

. 00689 

68.9 

• 7 1 

none 

111 . ■< Oran, lead, 60 gms. 

{ PbO 1 A.T. 

2/ 

49 

25 — 

. 00647 

trace 

.00005 

. 0065 2 

65 . 2 

. 00 

• 77 


Assays a, e, and h were all made at the same time and under conditions as nearly alike 
as possible. y 

Assays b, f, and j were all made at the same time and under like conditions. 


































ASSAY OF ORES FOR GOLD. 


145 


of gold in the slag, but the percentage loss seems to be less than 
that sustained in cupellation. 

“ As the ore carried very little silver, enough pure silver was 
added to all the lead buttons at the time of cupellation to part 
the resultant bead. It may be of interest to state that tests upon 
the bone-ash from which the cupels were made by us showed 
that 

« 

3.5% would remain on a 60-mesh sieve. 

15.9% would pass 60 and remain on 80. 

27.8% would pass 80 and remain on 100. 

52.8% would pass through 100-mesh sieve. 


CRUCIBLE METHOD. 



Charges. 

No. of Experiment. 

Weight of Lead. 

Wt. after Scorifying 
once. 

Wt. after Scorifying 
Twice. 

Wt. of Gold Obtained 
in Grammes. 

Wt. of Gold Found in 
Slag. 

Wt. of Gold Found in 
Cupel. 

Total Gold accounted 
for. 

Ounces per Ton. 

Per Cent of Total Gold 

Found in Slag. 

Per Cent of Total Gold 

Found in Cupel. 


I. 

Ore * A.T. 

Soda, 60 grammes. . 

1 

30 



•03391 

.00007 

.00008 

.03406 

68.12 

. 21 

.23 


PbO, 120 “ 

.Argols, 1 gramme.. . 

2 

27 

— 

— 

•03417 

.ocoi9 

lost 

•03436 

68.72 

.56 

-- 

Mix 

Borax glass cover. 

II. 

fOre i A.T. 

Soda, 20 grammes. . 

1 

80 

47 

21 

.03463 

none 

none 

.03463 

69.26 

none 

none 

PbO, 80 “ 

Gran. Pb, 50 “ 

2 

75 

5 i 

25 

•03387 

.00008 

.00012 

.03407 

68.14 

.24 

• 35 

W 4 

.Argols, 1 gramme.. . 
Cover of salt. 

III. 

fOre, * A.T. 

Soda, 40 grammes. . 

1 

29 



•03393 

trace 

.00008 

.03401 

68.02 


• 24 

a -i 

PbO, 60 

2 

25 

— 

— 

.03414 

.00005 

.00014 

.03433 

68.66 

.15 

.41 


Borax glass, 15 gms. 
Argols, 1 gramme.. . 

3 

25 

— 

— 

.03401 

trace 

.00016 

.03417 

68.34 

— 

.46 

Mix 

_ 

Cover of salt. 

IV. 

'Ore, 1 A.T. 

Soda, 60 grammes. . 

1 

25 



.06698 

.00009 

.00005 

.06712 

67.12 

.13 

• 07 

Borax glass, 15 gms. 
PbO, 60 grammes.. . 

2 

28 

— 

— 

.06820 

.00011 

.00015 

.06846 

68.46 

. 16 

.22 


.Nitre, 1 gramme. 
Cover of salt. 








Av. 

68.32 




Taking all the results between 68 and 69 oz. we have an average of 68.35 oz. 


“ From the foregoing tables it is very evident that the ore, 
setting aside the loss through volatilization, assays between 68 
and 69 ounces per ton, and any results below this are due to 



































NOTES ON ASSAYING. 


1^6 

the ore weighed out not being a correct or even sample of the 
whole. Of the eight scorification assays three are practically 
correct, and are within the limits of error of assay, with no cor¬ 
rection made for the gold found in the slag and cupel. The 
remaining five are all too low, even with the corrections made, 
and there seems no way for accounting for these low results 
other than that the portion of ore taken was an incorrect sample 
of the whole. 

“ The crucible results are much more even, and the percentage 
of loss sustained both in the slag and in cupellation lower than 
in the scorification method. As the amount of ore taken is 
five times larger in seven cases and ten times larger in two cases 
than in the scorification method, this may account for the more 
uniform results and the smaller percentage of loss sustained. 

“ As for the charges used, no comparison is drawn. Our only 
regret is that the sample of ore was so small that we were unable 
to carry out further experiments which the above results sug¬ 
gest. No doubt if the ore had been crushed to pass through a 
160- or 180-mesh sieve more uniform results would have been 
obtained in the scorification method. It was only a short time 
ago when a 6o-mesh sieve was considered sufficiently fine to 
pass any ore through previous to assaying. This was then set 
aside for an 8o-mesh, and now a ioo-mesh is generally used 
in most assay offices. I believe it only a question of time when 
every sample will have to pass a 140-mesh. What assayers 
need is a machine easily cleaned, which will grind ores through 
such a sieve quickly, and at the same time not contaminate the 
sample with the iron or material of which the machine is made. 
The foregoing experiments also bring up the questions of how 
close assay results should check, and what the percentage of loss 
is in assay work. 

“As to the first question, some ores will check easily, even if 
the ore is ground no finer than through a 60-mesh sieve, but 
these are the exceptions. With other ores, even when ground 
so fine that they will pass through bolting-cloth finer than 200 
meshes to the inch, it seems impossible to obtain anything like 
uniform results. 


ASSAY OF ORES FOR GOLD. 


*47 

“As to the percentage of loss sustained in work, whether by 
the scorification or the crucible method, many experiments 
carried out upon the foregoing lines, both upon silver and gold 
ores, indicate to me that nothing definite can be laid down in 
regard to it. Every ore, every slag, every scorification, and 
every cupel, let alone the temperature at which the assay is 
carried on, has some effect upon the loss, and these make too 
many unknown quantities to arrive at any definite conclusion.” 

The following additional data were obtained by Mr. C. E. 
Danforth, class of 1905. 

Two ores used, both through 170-sieve, analyzed as follows: 
No. 1. Si0 2 = 78.5%; Te = 5.i6%; FeS 2 , CaO, and H 3 P0 4 


were also present. R.P. =^. 

Gold, wet analysis. 287.7 oz. 

“ scorification assay. 287.02“ 

“ crucible assay. 287.9 “ 

Silver. 258.3 “ 


No. 3. Si0 2 = 74-5%; Te = 7.i%; FeS 2 , CaO, and H3PO4 
were also present. Gold 256.4 oz. Silver 826 oz. 

At the time of cupellation C.P. silver was added to each assay 
to diminish the loss of gold absorbed by the cupel. This amount 
added to that present in the ore made the ratio of silver to gold 
6 to 1. 

Different temperatures and many variations in the fluxes were 
experimented with. The dry and wet methods of analysis for 
gold were also compared. 

The results of this work seem to show: 

1 st. That uniform results are very difficult to obtain by any 
method on ores as rich in gold and carrying as much tellurium 
as these two do. 

2d. That the ore should be weighed on balances sensitive at 
least to .02 of a milligramme. 

3d. That when ores carry as much tellurium as these do, i.e., 
when a little ore gives a strong pink solution with H 2 S0 4 , no more 
than A.T. of ore should be used for any fire assay, otherwise tel- 






148 


NOTES ON ASSAYING . 


lurium will be present in the resulting silver and gold bead, giving 
it a rough and frosted appearance. 

4th. That in scorification work a large amount of lead and a 
high temperature (iooo° C. or more) are necessary. 

5th. That the addition of PbO in scorification helps to elim¬ 
inate the tellurium, but the silver and gold results are both low. 

6th. That in crucible work a large or small amount of litharge 
may be used. A high temperature seems advisable and especially 
high soda, but if the ratio of soda to both ore and litharge is 
very high (soda 30 grammes, litharge 25, ore A.T.), the tem¬ 
perature may be low. 

The ratio of bicarbonate of soda to ore varied from 3.4 to 1 
up to 20 to 1; the ratio of litharge to ore from 17 to 1 up to 31 
to 1, and the ratio of soda to litharge from 1} to 1 up to 1 to 9. 

The temperature varied from 890° to 1240° C. 

7th. That the addition of nitre does not prevent the tellurium 
from passing into the lead button. 

8th. That the presence of tellurium in a silver and gold bead 
aids the parting in H2SO4. The parting is more rapid and the gold 
is left in one piece in a spongy condition. This spongy appear¬ 
ance seems to be a very delicate test for tellurium, for in all 
cases where the H2SO4 was colored pink the gold was spongy, 
and in many instances, when a very slight amount of tellurium 
was present, it was spongy when the H2SO4 showed no trace of 
color. 

A third ore, carrying only a small amount of tellurium, gave 
no trouble, the results being very uniform. It would all pass 
through a 200-mesh screen, had a R.P. of .7, and assayed 2.60 oz. 
gold and 11.46 oz. silver. 

The following are some charges which have been given to 
me: 

Mr. G. A. Packard, class of 1890, for rich ores in San Juan 
district, Colorado, recommends a crucible assay with ± A.T. of 
ore, litharge 60 grammes, some soda, some potash, the necessary 
amount of reducing agent, and a cover of borax glass. 

Mr. C. S. Hiirter, 1898, recommends a crucible assay with 


ASS A Y OF ORES FOR GOLD. 


149 


i A.T. of ore and not less than 180 to 200 grammes of litharge, 
increasing the amount of the acid fluxes owing to the large amount 
of litharge. 

Mr. J. H. Batcheller, 1900, recommends the following: 

^ A.T. if the ore is rich. 


h A.T. “ “ 


u (( 


20 gms. of flux: < 


poor. 

Soda. 40 parts 

Potash. 20 “ 

Flour. 8 “ 

Borax. 10 “ 


- Mix well. 


50 “ PbO. 

5 “ silica. 

Cover with 30 grammes of borax glass. 

Heat at just as high a temperature as the muffle will give. 

In Cripple Creek, Colo., they take yV 1 ° i A.T. of ore, 2\ 
oz. of a flux made up somewhat as follows, and fuse in a muffle- 
furnace. The slag will be glassy and brittle. 


Flux. 


K 2 co,. 

. 3°7 

kilos 

1 

Na 0 CO 3 . 


C C 

Borax glass. 

. 2 - 5 S 

( ( 


Flour. 

. 45 

c c 

f 

J 

Litharge. 


( c 


Mix 

well. 


w 

a 


rt * 
h. 3 
<U Sh 

B 

d 

in 


r 


Ore £ A.T, 
10 

I § 

8 

i-3 

s. 41-0 


grammes 

i C 


( ( 


68.3 grammes or 
about 21 oz. 


Testing an Ore for Tellurium.—Take some of the finely 
pulverized ore and heat gently with strong H2SO4 in a white 
vessel. If tellurium is present, a faint purple tinge will be seen 
about the ore particles which will gradually spread through the 
solution, coloring it deep carmine if much is present. The cause 
of this color is doubtful; some say it is due to tellurous oxide, 
others to tellurium sulphite. It will disappear on boiling and 
upon the addition of water, which throws down the metal as 
a grayish-black powder (TeS 0 3 + H 2 0 =Te + H 2 S 0 4 ). If the 
ore itself gives no test, take 100 grammes or more of the ore 
and carefully pan it; then treat the concentrates as described. 

















NOTES ON ASSAYING. 


150 


The following are some references as to telluride ores: 

C- W. Fulton, School of Mines Quarterly, Vol. XIX. 

Mineral Industry, Vol. VI, Telluride Ores. 

Effect of Tellurium upon the Cupellation of Gold.—The fol¬ 
lowing work was done by Messrs. F .J. Eager and W. W. Welch: 


No. 

C.P. Gold. 

Lead, 

Grammes. 

Temp. 

C. 

Percentage 
of Tellu¬ 
rium Used. 

Percentage 
Gold Lost. 

Mean 
of the 
Two. 

I 

.20181 

IO 

775 ° 

none 

•15 


2 

.20104 

< < 

( < 

i C 

.16 

•155 

3 

.20025 

a 

U 

2-5 

. IO 

4 

.20408 

(i 

c c 

< < 

.19 

•145 

5 

•20334 

c ( 

C ( 

5.12 

.12 

6 

.20089 

(i 

i i 

( c 

.11 

• Ir S 

7 

.20392 

(i 

C ( 

7-5 

•17 

8 

• 20590 

i < 

(i 

< < 

•15 

.160 

9 

.20226 

(< 

t < 

10 

.12 


10 

.20263 

< ( 

(< 

< < 

.l6 

.140 


The results agree very closely with the ones obtained when 
cupelling pure gold (see page 160) and seem to indicate that the 
presence of 10% of tellurium has no influence on the loss of 
gold, which is entirely at variance with many published results, 
which show very high losses when tellurium is present in the 
lead button. All the buttons were bright yellow and showed 
no evidence of tellurium. 

The tellurium gave a pinkish color to the surface of the 
cupels, which, in great part, faded away upon cooling. 

Cupelling and Weighing Beads of the Precious Metals.—If the 
ore is to be assayed for both silver and gold, the button resulting 
from the cupellation is weighed previous to the parting with acid 
and the silver determined as follows: 

Silver and gold bead.= .00847 grammes in 1 A.T. 

Silver in 30 grammes of PbO used... = .00037 “ 

a 

u 


Silver and gold. . . 
Gold from parting 
Silver. 


=.00810 
=.00210 
=.00600 


u 


= 2.1 oz. 
= 6 oz. 























ASSAY OF ORES FOR GOLD. 


I 5 I 

There is sufficient silver in this button for parting; but if a 
button does not part in acid, silver has to be added. This silver 
need not be weighed. The final calculation is made as in 
example. 

When we do not care to know the amount of silver in the 
ore and the ore is known to carry a very small quantity of both 
silver and gold or a large quantity of gold and little if any silver, 
it is well to add a small piece of C.P. silver-foil to the lead but¬ 
ton at the time of cupellation. This will not only give a button, 
but will allow this button to be parted and save fusing it with 
silver afterwards. Students should bear in mind, however, 
that if the silver is in too great excess or in too large ratio to the 
gold, the button is apt to part too rapidly and the gold be finely 
divided, unless very weak acid is used. 

Parting.—The separation of gold from silver is called “ part¬ 
ing.” For this purpose use either nitric or sulphuric acid. The 
acids must be as pure as they can possibly be made, for if either 
one contains a small quantity of the other, gold will go into solu¬ 
tion. The HNO3 must also be free from HC 1 and free chlorine. 
It is claimed that when an alloy of gold and silver is in a thin 
plate, the best ratio in which to have the metals is between 2J 
and 3 parts silver to 1 part of gold. That is, an alloy of this 
ratio will part, leaving no silver in the gold, while an alloy con¬ 
taining less than 2J parts of Ag to 1 part of Au will not part. 

My experience has been that when an alloy is in a very thin 
plate, or a bead is small and has the ratio of silver 3 to gold 1, 
it may part and leave no silver or an extremely small amount 
in the gold, but in the ordinary run of work I believe it is advis¬ 
able to have the ratio from 6 to 10 of silver to 1 of gold. By 
having this ratio the button is sure to be parted and results in 
gold too high, owing to the presence of silver, will not be reported, 
which, in the case of beginners, is likely to occur if the ratio is 
3 silver to 1 gold. Furthermore, many recent tests seem to show 
that, by having this high ratio of silver present during cupella¬ 
tion, more .correct assays are obtained for gold due to the smaller 
absorption of gold by the cupel. 


NOTES ON ASSAYING. 



The danger of having the gold finely divided during the part¬ 
ing, owing to the high ratio of silver, can be easily avoided by 
using very dilute acid at first. 

Mr. T. K. Rose * recommends dropping the flattened beads 
into boiling nitric acid (sp. gr. 1.25) and says that “under these 
conditions the parting is rapid and complete and the bead hardly 
ever breaks up whatever its composition. Small beads, with 
much silver, part almost instantaneously, large beads in from 
five to ten minutes, and no second acid is required.” 

Inquartation or Quartation (i.e., one part in four).—This 
is an operation by which the alloy or button is brought to this 
standard or ratio. 

The button or alloy of gold and silver is carejidly cleaned , 
weighed , and hammered flat on a small anvil. If it is a large 
button or bullion, it is annealed repeatedly y~7\ an d rolled 
out gradually into a thin plate or ribbon. lUk it j s nex t 
rolled up into the form of a cornet or of a coil and then placed 
in a parting-flask. In parting buttons from ores some assayers 
prefer one thing, some another; small flasks, porcelain crucibles, 
or test-tubes may be used. 

The capacity of the flasks should be from 30 to 60 cc., and the 
^ lip should be round and not liable to break. The 
hX adjoining figures represent two forms of flasks. B 
B \ has the advantage over A in that the sides are 
— straight, and they both have the advantage over a 
test-tube in that the contents, when heated, are not so apt to 
bump. In France and in some mints they use a flask 
shaped as in the annexed figure. In parting we do 
not use strong HNO s (1.42 sp. gr.), for this seems to 
have some action upon gold. (If the acid in which 
the alloy has been parted turns yellow and then brown 
or violet upon the addition of water, gold has un¬ 
doubtedly gone into solution.) In some tests .12 to .15 per 
cent of gold went into solution. Acid of 1.13, 1.16 (18 0 Baumd), 
1.20, and 1.27 sp. gr. may be used. 




* Jl. Chem., Met. and Mg. Soc. of So. Africa, Jan. 1905. 






ASSAY OF ORES FOR GOLD. 


153 


At 65° Fah. 

Acid of 1.14 sp. gr. 


H 2 0 . 


HN °3 

2 * 1.42 sp. gr. 

s made up of 700 cc. and 260 cc. 



< < 

i i 

1.18 

i ( 

i i 

< < 

< i 

i i 

i ( 

700 

(< 

i ( 

380 

t 


( < 

< t 

1.194 

< < 

(( 

< i 

t ( 

< < 

< < 

0 

0 

< < 

(( 

420 

i 

24 ° 

Baume 

i ( 

1.208 

( ( 

( < 

( ( 

(1 

t ( 

< < 

700 

C i 

i ( 

470 

i ( 

28° 

«< 

i ( 

1.25 

i i 

< i 

i ( 

i i 

i < 

< < 

700 

< l 

i i 

700 

( 

30.6° 

< i 

i ( 

1.28 

< i 

t ( 

(( 

(( 

C i 

< < 

700 

(( 

1 i 

900 

i i 

32 - 5 ° 

i i 

i c 

1.30 

(i 

i < 

i i 

i i 

< i 

< < 

0 

0 

< < 

i < 

1100 

< 

32 - 5 ° 

( ( 

i ( 

1 - 3 l6 

i t 

< t 

i < 

i < 

i i 

i ( 

0 

0 

< ( 

( i 

1300 

i ( 


Place the bead, button, or strip of bullion in the flask or porce¬ 
lain crucible, add a little distilled water and then just enough 
acid (1.20 sp. gr.) to start action (the mints use acid of three dif¬ 
ferent strengths); heat gently, the object being to have the 
action take place slowly, and finally boil. If, after boiling a few 
minutes, no action is noticed upon the button and it appears 
round and hard, there is probably not sufficient silver present in 
it. Wash with distilled H 2 0 and transfer to an annealing-cup, 
dry and weigh. Add more than three times the weight of C.P. 
silver, wrap in C.P. lead, and cupel on a fresh cupel. 

If upon boiling in the dilute acid there is action upon the 
button, continue the boiling until action almost ceases, decant 
the solution containing the silver nitrate (save this AgN0 3 in a 
bottle), add a fresh portion of acid (1.20 sp. gr.), and boil again. 
Repeat a third time and either add a little 1.40 acid to the 1.20 
or else boil in 1.27 sp. gr. acid until there is no action or until 
the last traces of the silver are removed from the button or cor¬ 
net. The acid will usually boil where the gold particles are; 
do not mistake this for the solvent action of the acid upon the 
silver. If there is any doubt about this or if the contents of 
the flask are inclined to bump, place a light stirring-rod in flask, 
when the boiling will take place chiefly about this. If the student 
suspects that the ratio of the silver to the gold is very large, have 
the solution of the silver take place all the more slowly, so as 
to keep the gold in one piece. 

The boiling tends to collect the particles of gold and removes 
any air from the fine flakes. Finally rotate the flask gently, 















1 54 


NOTES ON A SAYING. 


decant the solution, and wash with hot distilled water two or 
three times. When decanting the solution or water always hold a 
piece oj white paper beneath the flask, so that you can watch the 
gold residue. 

The gold is then ready to be transferred to a clay or porce¬ 
lain annealing-cup or to a porcelain crucible. The clay annealing- 
cups or dry cups should have round smooth edges and not sharp 
ones. The Battersea forms A and B cannot be improved upon. 

The clay cups have the advantage over the glazed porcelain 
ones in that they are porous and can absorb some water and give 
it off slowly. They have the disadvantage that, if not carefully 
used, some of the material from the cup or cover may break or 
rub off and get into the gold. The porcelain cups have the advan¬ 
tage of the surface being perfectly smooth and glazed, so that it 
cannot be rubbed off and get into the gold. They have the 
disadvantage of breaking more easily upon heating and cooling 
and of being apt to spatter in drying. The latter can, however, 
be almost entirely obviated in the following way: Have only a 
little water left in the cup, then add some absolute alcohol and 
set this on fire. By the time the alcohol has all burned, the water 
will have evaporated and the gold will be left in the cup in a con¬ 
dition to stand the full temperature of a lamp or muffle. When 
the porcelain crucibles are hot, use hot tongs with which to handle 
them. 

The next step in the parting process, if a flask or test-tube 
has been used, is to fill it jull to the edge with distilled water, 
place an annealing-cup or porcelain crucible on top, and invert 
quickly; allow the gold to settle into the cup, shake the flask, 
tap it and rotate it occasionally; raise the 
flask gently and allow air to enter slowly, 
but do not allow it to disturb and break 
up the gold. When the mouth of the 
flask is even with the top of the cup and 
the latter is full of water, slide the flask 
off quickly at right angles to the cup, 

drain the water from the cup as far as possible, cover it up, and 
dry it upon an iron plate. 









ASSAY OF ORES FOR GOLD. 


*55 

The gold will be in the form of a dark-brown or black powder- 
Finally, heat the cup in a muffle-furnace or over a Bunsen lamp- 
until it is red on the bottom and the gold is bright yellow. 

During the whole process of “parting” the student must be* 
very careful to get no foreign matter of any kind into the flask or 
the annealing-cup. Never pass one flask over another nor one- 
annealing-cup over another, as dirt may fall off one into the other- 
If the student heats the clay annealing-cups in a muffle, which is. 
the most satisfactory method, have them dry before placing them 

in the muffle and do not touch the- 
^— muffle or furnace in any way during; 
the heating. Handle the cups care¬ 
fully with tongs, similar to those represented in the figure, and 
keep the covers on them until ready to weigh the gold. 

When the cup and its contents are perfectly cold, have the 
fine balances (sensitive to 1 / 100 of a milligramme, i.e., .00001 
gramme) in perfect adjustment, remove the scale-pan from the 
balances, and transfer the gold from the cup to the pan. Do 
this by tapping gently the side of the annealing-cup. This should 
detach any small particles of gold which may be adhering there, 
and then by tipping the cup slowly over the pan all the gold will 
slide nicely from the cup into the pan. Any particles adhering to 
the cup may be detached by means of a small feather trimmed to 
a fine point. Replace the scale-pan and weigh as accurately as 
possible. Report the result in ounces as follows: 

Ore used =1 A.T. 

Gold as weighed = .00126 grammes. 

= 1.26 oz. per ton of 2000 lbs. of ore. 

Value @ $2o 67 /ioo P er oz * (U. S. Standard) = $26 04 / 100 

After weighing the gold always examine it carefully not only 
to see whether any foreign matter has been weighed with it, but 
also to see whether it is pure yellow. If it has a white appearance* 
it has not parted and contains silver; if it is dark or black, it 
probably contains some of the rare metals of the Pt group. 

Gold cupelled with bismuth almost always retains traces of 





NOTES ON ASSAYING . 


156 

this metal, and very small amounts of it or of lead may make the 
gold brittle and non-malleable. 

The “flashing”* of a gold button, i.e., when in cooling it 
emits a brilliant, clear, greenish light, is said to indicate its purity; 
and if this takes place, Ir, Rh, Os, Ru, and Oslr must be absent, 
for extremely small quantities will prevent it. 

On ordinary ores results in gold should agree within .02 oz. 
Assays of seller and buyer should check within .04 oz. 

If the seller finds 1.42 oz. and the buyer 1.38 oz., the ore is 
settled for at 1.40 oz.; if the seller finds 1.42 and 1.44 oz. and 
the buyer 1.35 and 1.36 oz., a sample of the ore is sent to an 
umpire, who makes from two to four assays. 

Generally, the mean of these assays is taken. 

Suppose the umpire finds 1.40 oz., then the ore is bought on 
that basis. 

If he finds 1.46 oz., then the settlement is based on 1.44 oz. 
If he finds 1.34 oz., then 1.36 oz. per ton is the basis. 

Smelters generally pay $19.50 to $20 per ounce for all gold 
present above .05 of an ounce. 

* Flashing in Assays of Gold. Dr. A. D. Van Biemsdijk, Chem. News, vol. 41, 
pp. 126 and 266. 






ASSAY OF ORES FOR GOLD. 


J 57 


Separation of Gold from Platinum and Iridium: Wet Method.* 

“The method consists in treating the solution containing the 
metals with 10-15 cc - of peroxide of hydrogen after the addition 
of about 5 cc. of an alkaline lye (KHO or NaHO, 480 grammes per 
litre). While other methods require several hours tc effect a com¬ 
plete reduction, in this case the gold is precipitated in a few 
minutes, even in the cold, as a black deposit, which under the 
action of heat agglomerates and becomes of a reddish-brown 

color: 6 

2 AuC 1 3 + 3HA+6KOH = 2A11 +60 + 6 K.C 1 + - 

In case of dilute solutions it is best to apply heat after the 
precipitation, then acidulate with HC 1 . For the estimation of 
gold in commercial chloraurate of sodium it is, however, prefer¬ 
able to effect the reduction by means of formic aldehyde instead 
of hydrogen peroxide. 

The reaction of peroxide of hydrogen in alkaline solution is 
much more sensitive qualitatively than any other reaction of 
gold. With 3 milligrammes of gold per litre we can still perceive 
a pale reddish coloration, appearing blue by reflected light; this 
would not be detected by other reagents. 

Silver is also precipitated quantitatively under the same con¬ 
ditions, but platinum as well as iridium remains in solution; 
this affords an excellent method for separating these two metals 
from gold.” 

EXPERIMENT: ROASTING CONCENTRATES OR AN ORE CARRY¬ 
ING GOLD. 

The objects of the test are: 

1st. To find the assay and value of the concentrates or of the 

ore. 

2d. To ascertain the loss in weight of concentrates or of the 
ore during the roast. 

3d. To assay the roasted material to discover the loss of gold, 
if any, during the roast. 

4th. To see how good an assayer one is. 

* Chem. News, vol. 82, p. 70. Estimate of gold and its separation from Pt 
and Ir. L. Vanino and L. Seemann. 




158 


NOTES ON ASSAYING. 


Take the concentrates resulting from the panning test 
(through 30- or 40-sieve) or some that will be assigned to you. 
Mix very thoroughly and with a broad spatula take a sample of 
200 grammes. Roll this well on the sampling cloth or paper; 
take with a broad spatula 90 grammes (weigh on flux-balance), 
crush it through a 120-sieve and assay for gold. Make two 
assays, using 1 A.T. in each case, unless the ore carries copper or 
has a reducing power of 4J or more, when use only J A.T. Add 
silver to every assay unless ore is known to carry sufficient silver 
jor parting. 

Weigh out exactly no grammes of the ore (through 30- or 
40-sieve) on the pulp-balance and roast carefully in a clay dish 
in a muffle as per Class II, D, page 132, also page 214. 

See that the clay dish will go into the muffle and have only 
sufficient juel in the jurnace to come up to the bottom oj the 
muffle. 

Use every precaution to avoid mechanical loss of the ore. 
Do not heat so fast that the ore will decrepitate, and after stirring 
the ore each time always hit the iron stirring-rod on the dish to 
shake of) any ore that may adhere to the rod. 

Roast the ore dead, i.e., so that neither sulphides, sulphates, 
arsenides, nor arsenates are present in it. 

Weigh the roasted ore on a pulp-balance to the second place 
■ of decimals, and calculate the per cent of ore lost during the 
_ roast. 

This loss depends entirely upon the composition cj Tie ere 
or concentrates , and may be very slight or as much as 40 per cent. 

Grind the roasted ore through 120-sieve and assay for gold. 
(See page 129, examples 3, 5, and 6.) 

As the ore generally loses weight, the assay of the roasted 
ore must necessarily be higher than that of the raw ore, unless 
there has been a heavy loss in gold. 

Report as follows: 

Example. —Concentrates from Ore No. 2444. Through ^o- 
mesh sieve. Consist of arsenopyrite, pyrite, and a little slate. 

Took 200 grammes of concentrates. 


ASSAY OF ORES FOR GOLD. 


159 


Assay showed 1.12 oz. per ton of 2000 lbs. 

Ore taken for roasting = no grammes 
Ore after roasting =86 “ 

Loss 24 “ =2I T 8 F % 

Total gold in no grammes = 29.16:no:00112 \x—. 00422. 

Roasted ore assays 1.40 oz. gold per ton. 

Total gold in roasted ore is 29.16:86: .'.00140:#=.00413. 

Gold lost (.00422— .00413) = .00009 = 2.13% 

The foregoing experiment is a most valuable one in many ways, 
for it shows how carefully a student works, how good an assayer 
he is, and how much he has profited by his previous work. 

Gold volatilizes easily before an ordinary blowpipe, giving a 
purple stain (oxide of gold); to prove this, hold a moist vessel 
over the charcoal or cupel, condense the fumes, dry, and assay the 
residue. (See Napier’s experiments.*) 

The metal in the mint, when in crucibles ready to pour for 
coinage, is said to have a temperature of noo° to 1150° C. 
Metals when in a melted condition absorb a great deal of gas, and 
gold is no exception.! According to T. K. Rose, an atmosphere of 
CO apparently increases the volatility, and he says that the loss 
in clay crucibles is less than in cupels, and less in the latter than 
it is in graphite crucibles. 

The following results and those given on page 150 are of interest 
in this connection. 

It will be remembered that silver can be cupelled successfully 
at 700° and even below, but this cannot be done in the case of 
gold. The loss in gold increases gradually with the temperature 
until the neighborhood of iooo° is reached, when it increases 
rapidly. In these experiments the loss at this temperature was 
chiefly due to minute buttons found on the inner surface of the 

* Volatilization of Metallic Gold. Journal of Chem. Soc., vol. io, p. 229, by 
James Napier. T. K. Rose, J. of Chem. Soc., vol. 63, p. 714. 

f Phil. Trans., 1866, 399-439, by Graham. 





160 NOTES ON ASSAYING. 

cupel. At first it was thought the large buttons had sprouted, 
although they presented a smooth surface. The following experh 
ment was then tried: Three lots of gold were cupelled side by side 
at iooo° C.; one was withdrawn when the io grammes of lead were 
about half cupelled, another when nearly at the point of blicking, 
and the third after the button had blicked. All the cupels had 
small gold buttons scattered over them, but none of the buttons 
were observed to spit. On the one first withdrawn the gold 
buttons were small and few in number; on the second there 
were more buttons and larger ones, and on the last a good many 
quite large ones were found. 

CUPELLATION OF GOLD AT DIFFERENT TEMPERATURES TO 

DETERMINE LOSSES. 


Experiments by Messrs. F. J. Eager and W. W. Welch. 


No. 

Gold. C.P. 

Lead, 

Grammes. 

Temp. 

C. 

Per Cent 
Lost. 

Mean of the 
Two Nearest 
Together. 

I 

.20026 

IO 

O 

O 

O 

All three of 

these buttons 

2 

.20176 

ii 

tt 

froze owing 

to the temper- 

3 

.20421 

<( 

tt 

ature being 

too low. 

4 

.20181 

tt 

775 ° 

.15 


S 

.20104 

a 

tt 

. 16 

•155 

7 

.20168 

tt 

850° 

.40 

8 

.20047 

tt 

it 

•55 


9 

•20153 

tt 

ii 

•37 

•385 

10 

.20513 


925 0 

• 45 


ii 

.20353 

tt 

ii 

.46 


12 

.20398 

tt 

it 

.46 

.460 

13 

.20180 

tt 

IOOO 0 

1.44 


14 

.20120 

tt 

ii 

1.28 


i 5 

.20166 

tt 

a 

1 -45 

i -435 

1 7 

18 

.20251 
.20173 


1075° 

a 

3-34 

2.64 

2.990 


Where the cupel was most eaten into there the larger number 
of buttons were found, and the softer the cupel the more it was 
eaten. Hard cupels were also attacked, and where eaten there 
buttons were found, which seems to indicate that the higher the 
temperature the more the litharge attacks the cupels, for no 
small buttons were found on the same quality of cupels run 
below iooo° C. At high temperatures, for some reason (perhaps 
less capillary attraction of the lead), small particles of the alloy 
are left behind and cupel by themselves; therefore gold but- 


















ASSAY OF ORES FOR GOLD. 


161 


tons should not be cupelled above the neighborhood of 8oo° C. 
It was noticed in these tests, as in the case of the cupellation of 
silver (page 63), that as the loss of gold increased the color of the 
litharge in the cupels became more green. To determine whether 
this green color had any significance, a gold button weighing 
1.4 grammes was cupelled at an ordinary temperature and the 
cupel was quite green. On assaying this cupel .00154 grammes 
of gold were recovered, showing a loss of .11 per cent by absorp¬ 
tion. 

The following are other examples: 


Gold Button 
obtained. 


Gold found Per Cent 

in Cupel. absorbed. 


3.2550 grammes .00390 grammes udoo 

3.2680 “ .00462 “ 14 Aoo 


The following table shows the effect of copper on the cupella¬ 
tion of gold. 

Lead, gold, and temperature constant, copper varying. 


No. 

Gold 

C.P., 

Grammes. 

Lead, 

Grammes. 

Copper. 
Per Cent of 
the Gold. 

Temp. 

c. 

Per 

Cent 

Loss. 

Mean of 
the Two 
Nearest. 

Ratio of 
Lead to 
Copper. 

1 

.20181 

10 

none 

775 ° 

•15 



2 

.20104 

it 

U 

a 

. 16 

.155 


4 

.20288 

a 

Ln 

NO 

o\ 

a 

.18 


1000 to I 

5 

.20110 

it 

a 

a 

. 20 

.19 

it 

6 

.20318 

tt 

a 

a 

. IO 


it 

7 * 

.20102 

a 

10% 

i i 

.20 


500 to I 

8 

.20142 

tt 

a 

a 

. 20 


it 

9 

.20138 

a 

a 

a 

. 20 

.20 

it 

10 

.20024 

it 

15% 

a 

. II 


333 to 1 

11 

.20060 

tt 

a 

a 

.26 


it 

12 

.20048 

a 

a 

a 

•15 

.13 

a 

13 

.20100 

ct 

20% 

a 

•13 


250 to I 

14 

.20101 

a 

a 

a 

■56 


a 

15 

.20161 

a 

<c 

a 

. 20 

. 165 

a 

16 

.20422 

a 

25% 

a 

.28 


200 to I 

17 

.20296 

a 

a 

a 

. 2 1 

.25 

( C 

18 

.20284 

a 

a 

a 

•31 




* Buttons 7-18 gained in weight. 


Many very interesting things are shown in this series of cupel- 
lations. All the gold beads showed the presence of copper except 



































162 


NOTES ON ASSAYING. 


the first series, in which 5 per cent was used. When 10 per cent 
was used the amount left in the button w T as a little more than 
the usual gold loss (.16%) at 775 0 , so the buttons, after cupellation, 
were practically the original weights. All the buttons blicked, 
and even in some tests, in which 50% of copper was used, a fair 
blick was obtained. 

The tests show with the high ratio of 1000 of lead to 1 of cop¬ 
per that 5 per cent of copper will be oxidized during cupellation, 
for the gold loss in that series was about normal. 

These results are contrary to what Napier found, who says 
that ‘‘the greater the amount of copper and the greater the heat, 
the more gold is then lost, and that gold, when alloyed with copper, 
is more volatile than when alone.” 

T. Iv. Rose says: “If the proportion of copper is increased, 
more gold is absorbed by the cupel.” 

Effect of Increasing the Ratio of Silver to Gold upon the Loss 
of Gold. —Some experiments given in the first edition of these 
notes seemed to show that the loss of gold during cupellation 
was not diminished by the addition of ten or more times as much 
silver to the assay as the amount of gold supposed to be present 
in the ore. 

Further experiments indicate that the addition of silver in large 
excess does lessen this loss of gold. 

Experiments at the Royal Mint in England show that there 
is almost always a small amount of silver left in the gold after 
parting, that is, about .09 per cent. This is called the surcharge.* 

If strong nitric acid (1.42 sp. gr.) is used, this amount of 
silver may be slightly decreased, but the gold will begin to dissolve. 

Parting with Sulphuric Acid. —When sulphuric acid is used 
for parting it is said that less silver is left in the gold, and no 
gold is dissolved. My experience is that no gold is dissolved, 
but silver is more likely to be left undissolved than in the treat¬ 
ment with nitric acid. The acid must be boiled a long time 

* Surcharge, as defined by T. K. Rose, “is the algebraical sum of the losses of 
gold sustained during the various operations and the amount of foreign substance, 
^chiefly silver, left in the gold cornet when weighed. 



ASSAY OF ORES FOR GOLD. 163 

and even then silver may be retained by the gold. There are 
also the following disadvantages in its use. 

1. It must be used full strength, which renders it liable to 
bum”' violently when boiled. 

2. Lead and platinum are not dissolved. 

3. Difficulty of washing the gold, which must be done very 
carefully. 

4. Sulphate of silver is not very soluble in water; so if much 
silver is present the first washings must be made with dilute sul¬ 
phuric acid. 


SPECIAL METHODS. 

ASSAY OF ZINC-BOX RESIDUES FROM THE CYANIDE PROCESS.* 

“ Several methods, both wet and dry, for the assay of zinc- 
box residues from the cyanide process, have been described in 
recent years, and each of them has been claimed to be superior 
to all others. In the year 1901, a paper, entitled ‘Assay of 
Zinc Precipitates,’ was published in the School 0) Mines Quar¬ 
terly to the purport that the scorification method for the assay 
of zinc-box residues was absolutely unreliable. 

In order to shed light on this matter, the following experi¬ 
ments were undertaken by Messrs. C. B. Hollis and F. D. Kehew, 
undergraduate students at the Massachusetts Institute of Tech¬ 
nology. 

The zinc-box residues used were obtained through the cour¬ 
tesy of Mr. H. R. Batcheller. The samples were very rich and 
varied greatly in the fineness of their condition. 

Scorification Assays.—In the preliminary tests, the charge, 
which was weighed on a chemical balance, consisted of 0.1 A.T. 
of residues mixed with from 30 to 35 grammes of test-lead and 
placed in a 3-in. scorifier, over this an additional quantity of 
test-lead (from 30 to 35 grammes) was placed for a cover, and 
borax glass, varying in quantity from 3 to 15 grammes, was 
sprinkled over the top of each charge of the various assays for a 
cover. The charges were scorified in a muffle-furnace heated 


* Transactions American Institute of Mining Engineers, October, 1903. 



164 


NOTES ON ASSAYING. 


to the ordinary temperature which is used in scorification; in 
some cases the door of the muffle was left open, while in others 
it remained closed. Generally, the charges spit badly, espe¬ 
cially in the assays that were made with door of the muffle left 
open, or in those in which the door was opened too quickly. 
The results of the preliminary assays showed: 1. That in order 
to obtain approximately uniform results, the material submitted 
to the assay must be in sufficiently fine condition to pass through 
a 200-mesh sieve. 2. That the ordinary chemical balances are 
not sufficiently delicate to afford accurate results in handling 
these residues, which are so rich in gold and silver. 3. That a 
large quantity of borax glass is absolutely necessary (from 3 to 10 
grammes for 0.1 A.T. of residues); and 4. That spitting can 
be avoided, provided the muffle be heated to a high tempera¬ 
ture before the introduction of the charge, and provided the 
door of the muffle be kept closed until the contents of the scorifier 
have become thoroughly liquefied; after this the temperature 
may be lowered. 

The quantity of zinc-box residues received amounted to 
458 grammes, and, upon sizing, it was found that 146 grammes, 
or 31.8 per cent, remained upon a 125-mesh screen; 64 
grammes, or 13.9 per cent, passed through a 125-mesh screen 
and was caught on a 160-mesh screen; and 248 grammes, or 
54.1 per cent, passed through a 160-mesh screen. The entire 
quantity of residues was then put on a 160-mesh screen, and 
the material that sifted through was treated on a 200-mesh 
bolting-cloth, yielding 290 grammes of very fine material, less 
than 200-mesh in size, on which the tests were made. 

In order to mix the sample thoroughly, the entire quantity 
of fine material was placed in a 38-oz. bottle closed with a 
glass stopper and steadily shaken for 20 minutes, the bottle and 
its contents being alternately shaken and rotated. The mixed 
material was then poured out upon a glazed paper, on which it 
was rolled 100 times, finally being spread out in a thin layer 
covering an area 18 in. square. Spatula samples to the num¬ 
ber of 450 were then taken, which constituted a new sample, 
weighing 102 grammes. A chemical analysis of the new sam- 


ASSAY OF ORES FOR GOLD. 165 

pie showed that it contained 9.09 per cent of copper and 14.3 
per cent of zinc. 

The assays were made in a muffle-furnace heated with coke, 
and the cupels used were of the ordinary bone-ash variety made 
at the Institute of Technology. Ninety per cent of the mate¬ 
rial forming the cupels was of sufficient fineness to pass through 
an 80-mesh screen. 

Four charges, Nos. 1, 2, 3, and 4, each of 0.05 A.T. in 
weight, were weighed on an assay balance sensitive to 0.02 of 
a milligramme and treated as follows: 

No. 1. 0.05 A.T. of the residues was mixed with 35 grammes 
of test-lead in a 3-in. scorifier; 30 grammes of test-lead were 
then added to the top of the charge, followed by a cover of 10 
grammes of borax glass. 

No. 2. The same as No. 1. 

No. 3. 0.05 A.T. of the residues was mixed with 6 grammes 
of litharge in a 3-in. scorifier. Additional test-lead was added, 
amounting to 40 grammes, followed by a final cover of 10 grammes 
of borax glass. 

No. 4. 0.05 A.T. of the residues was mixed with 1 

gramme of fine charcoal and 35 grammes of test-lead; the 
mixture was then covered with 30 grammes of test- 
lead and a final cover of 10 grammes of borax glass. 

The scorifiers were placed in the muffle as shown in 
Fig. 1. The muffle was very hot and the door was 
kept closed for about 5 minutes, after which it was 
opened. 

Charge No. 1 spit badly, doubtless due to its position in the 
muffle; charge No. 3 spit to a slight extent; while charges 
Nos. 2 and 4 did not spit. Charge No. 4 became covered over 
very quickly, owing to the charcoal in the mixture, but the 
resultant button of lead was so large that it was necessary to 
rescorify it. Although zinc ores require a high temperature 
for fusion, the heat was lowered as soon as possible after 
the muffle was opened, in order to slag off the copper and 
avoid a second scorification. The fused material poured well 
and the color of the scorifier indicated that the buttons could 


Fig. i. 


4 

3 

2 

1 

Front. 



NOTES ON ASSAYING. 


166 

be cupelled with safety. Both the slags and the cupels were 
assayed by the crucible method, the results being given in 
Table I. The silver and gold beads from the slag of charges- 
Nos. i and 2, and those from the cupels used in tests Nos. 2 
and 3, sank into the cupels which were reassayed. This addi¬ 
tional assay may account for the low results in silver and gold 
that were obtained in tests Nos. 2 and 3. 

The silver-gold beads were weighed, but, as they did not 
contain sufficient silver to part them, they were recupelled, 
with the addition of chemically pure silver; the cupels of this, 
latter cupellation were not reassayed. The parting was done 
with nitric acid of 1.16, 1.20, and 1.27 specific gravities. The 
results obtained are given in Table I. 

Charges Nos. 5, 6, and 7, which were similar in all respects 
to charges Nos. 1 and 2, were next weighed and placed in the 
muffle as indicated in Fig. 2. The temperature of 
the muffle was that used in the ordinary assay, and 
the door of the muffle was kept closed for 10 min¬ 
utes. It was then opened and charge No. 6 was. 
seen to spit twice; the door was then closed, and, 
through an opening in the muffle, charge No. 6 was 
seen to spit a third time. The door of the muffle was then 
opened and the scorification completed. The slags and the 
cupels from these charges were assayed as in the former tests,, 
the results being given in Table I. 

In order to ascertain whether a scorifier of larger size would 
be beneficial or not, charge No. 8 was assayed in a 3-in. scori¬ 
fier, and charge No. 9 in a 4-in. scorifier. Charge No. 8 con¬ 
sisted of 0.05 A.T. of residues placed in the bottom of a 3-in., 
scorifier and covered with 65 grammes of test-lead, followed 
with a final cover of 10 grammes of borax glass. Charge No. 9 
consisted of 0.05 A.T. of residues, prepared as in charges 
Nos. 1, 2, 5, 6, and 7, with the exception that a 4-in 
scorifier was used in place of a 3-in. one. The tests were placed 
in the muffle as shown in Fig. 3. The muffle was closed and 
both charges were seen to spit badly. They were then allowed 
to become covered and were later poured and treated in a manner 


Fig. 2. 

5 

6 

_ 7 

Front. 



ASS A Y OF ORES FOR GOLD. 


167 


similar to the earlier tests. From the results of these tests, which 
are given in Table I, charges Nos. 8 and 9 were 
rather peculiar. The gold in No. 9 was very low, the 
silver was very high, and an exceedingly large quantity 
of silver was recovered from the slag. Charge No. 8, 
also, showed a high percentage of silver in the slag. 

The cause of these odd results was not apparent. 

Charges Nos. 10, n, and 12 were then made as follows: 

Charge No. 10 consisted of 0.05 A.T. of residues, 60 grammes 
of test-lead, 10 grammes of litharge, and 12 grammes of borax 
glass, thoroughly mixed together in a 4-in. scorifier. 

Charge No. n consisted of 0.05 A.T. of residues mixed with 
30 grammes of test-lead; on this were placed an additional 
30 grammes of test-lead, followed by a cover of 10 grammes 
of litharge and a final cover of 12 grammes of borax glass. 

Charge No. 12 consisted of 0.05 A.T. of residues mixed with 
30 grammes of test-lead in a 4-in. scorifier, over which were placed 
30 grammes of test-lead and a final cover of 14 grammes 
of borax glass. Charges Nos. 10, n, and 12 
were placed in the muffle as shown in Fig. 4, and 
the door of the muffle was kept closed for 10 min¬ 
utes. Charge No. 10 fused very quietly and did not 
even tend to jump. Charge No. n was less quiet, 
but did not spit, and charge No. 12 was quiet. The 
charges were allowed to become covered and were then poured* 
the resultant buttons, slags, and cupels being assayed as in the 
former tests. (See Table I.) 

The results from charges Nos. 11 and 12 were low, and the 
gold obtained from charge No. 10 was especially so. The silver- 
gold button obtained from the assay of the cupel used for charge 
No. 10 sank into the cupel, which had to be reassayed. 

Charge No. 13 was similar to charge No. 10, and consisted 
of 0.05 A.T. of residues, 60 grammes of test-lead, 10 grammes 
of litharge, and 12 grammes of borax glass, all thoroughly mixed 
together in a 4-in. scorifier. 

Charge No. 14 consisted of 0.05 A.T. of residues and 30 
grammes of litharge, mixed together in a 4-in. scorifier, and 


Fig. 4. 
12 

11 

10 _ 

Front. 


Fig. 3. 

8 

9 _ 

Front. 




i68 


NOTES ON ASSAYING. 


covered with 30 grammes of test-lead, with a final cover of 12 
grammes of borax glass. 

Charges Nos. 13 and 14 were placed in a hot muffle in the 
position shown in Fig. 5. The muffle was then closed for 10 
minutes. Charge No. 13 fused quietly and had no 
tendency to spit, while charge No. 14 spit several 
times after the door of the muffle was opened. These 
charges were treated in a manner similar to the pre¬ 
vious tests, except that, in the assay of the cupel of 
test No. 14, trouble was encountered with the silver- 
gold button, which accounts for lack of results given under this 
heading in Table I. The results for gold in tests Nos. 13 and 14 
were low, an effect which seems to be true of all assay charges 
containing litharge. 

A complete summary of the data obtained in tests Nos. 1 to 
14, inclusive, are given in Table I; and in Table II are given 
the weights of the lead buttons and other data relative to the 
assays of the slags and cupels of these tests. 

A study of the results shows that the addition of charcoal 
to the charge seems to aid the scorification. Also, that all charges 
in which litharge was used (Nos. 3, 10, n, 13, and 14) gave low 
results for gold. The addition of the litharge, however, seemed 
to prevent the spitting of the charge during fusion. 

The value of the residues in gold lies evidently between 4690 
and 4698.4 oz. per ton, as the results from 6 of the 14 charges 
are within these limits, and 3 of these 6 are practically identical, 
i.e., 4694 oz. The results for silver in 4 of the tests were between 
4175.2 and 4179.6 oz. per ton; 4178.5 oz. per ton being taken 
for the quantity present. 

Estimating the value of gold at $20.67 P er oz - an d silver at 
$0.50 per oz., the value per ton of the residues was: for gold, 
$97,025, and for silver, $2,089. On the basis of these values, a 
comparison of the highest and lowest content of gold as deter¬ 
mined by the scorification method and the percentage of varia¬ 
tion from the correct value is:— 

Ounces above, 4.4, value $91, or 0.09 per cent. 

Ounces below, 96.8, value $2,000.85, or 2.06 per cent. 


Fig. 5. 

13 

£4 _ 

Front. 



TABLE I.—DATA OF SCORIFICATION-ASSAYS OF ZINC-BOX RESIDUES CONTAINING COPPER, 9.09 per 

CENT., AND ZINC, 14.3 PER CENT. 


ASSAY OF ORES FOR GOLD 


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ASSAY OF ORES FOR GOLD . 


171 

The extremely low results of test No. 10 (54.8 oz., valued at 
$1,132.71, or 1.16 per cent) are omitted from this calculation. 

Summarizing the results of the determinations of silver in a 
similar manner, there were obtained the following figures: 

Ounces above, 495.5, value $247.7, or 11-85 P er cent. 

Ounces below, 30.1, value $15, or 0.72 per cent. The high 
results obtained in test No. 9 (87.9 oz., valued at $43.95, or 2 - 1 
per cent) were not included in this calculation. 

The results for the silver determinations were less uniform 
than were those for the gold, but the value is apparently between 
4175 and 4220 oz. per ton, 9 results out of the 14 being within 
this range. The charges containing litharge gave low results 
for silver in 2 out of 5 cases. 

The quantity of go ] d found in the slags was generally less 
than that found in the cupels—a result which is unusual. 

From the results obtained in tests Nos. 1 to 14, the best method 
for the treatment of these zinc-box residues is as follows:—The 
charge should consist of 0.05 Assay Ton of residues mixed with 35 
grammes of test-lead in a 3 or a 4-in. scorifier and covered with 
a layer of 30 grammes of test-lead, followed by a final cover of 
from 10 to 12 grammes of borax glass. The filled scorifier should 
be placed in a hot muffle (in order that the fusion shall occur 
rapidly), and the door should be closed for fully 5 minutes after 
the charge has been fused. During the time that the door is 
closed, no air whatever should enter the muffle. When the charge 
has become thoroughly fused, the door of the muffle should be 
opened and the remainder of the assay conducted in the usual 
manner. 

Confirmatory Wet Assays.—In order to confirm the results 
obtained in the scorification assays, Mr. Hollis made a duplicate 
determination of gold and silver in the zinc-box residues by the 
wet method of Mr. C. Whitehead. These results (charges 
Nos. 15 and 16) were respectively, for gold = 4698.8 and 4694.8 
oz. per ton; and for silver = 3841.6 and 3555.6 oz. per ton. 
The results for gold are practically the same as those obtained in 
the scorification assay. The results for the silver, however, 
are very much lower and are doubtless due to the incomplete 


1 7 2 


NOTES ON ASSAYING. 


precipitation of the silver bromide which is soluble to a certain 
extent in too strong a solution of potassium bromide. This 
effect is analogous to the action of silver chloride, for if a solution 
of silver nitrate be precipitated by salt in a solution that is not suffi¬ 
ciently diluted, all of the silver chloride will not be thrown down, 
some of it being dissolved in the strong brine. 

The residues were assayed also by the wet method suggested 
by Messrs. Charles H. Fulton and C. H. Crawford,* which is 
called the ‘combination wet and dry method of assay.’ 

The method was used exactly as described, with the excep¬ 
tion that the filter and content were not scorified, but were as¬ 
sayed in a glazed crucible after having been separately burned. 
The data obtained by the combination wet and dry method 
are given in Table III. 

The data given in Table III show that the results for gold 
were somewhat lower than those obtained by the scorification 
assay, while those for silver were very much lower. 

Crucible Assays.—Three portions of residues were taken, of 
0.05 A.T. weight, and to each were added 15 grammes of soda, 10 
grammes of borax glass, 90 grammes of litharge, and 2 grammes 
of argols; an excessive quantity of litharge was used, in order 
to slag the copper and the zinc. The fusion was made in a ‘G’ 
crucible, which had previously been glazed with borax glass, 
and each charge was fused for 35 minutes. One charge ate 
through the crucible, one would not pour, the third only seeming 
satisfactory. The results of the good test are given in Table IV, 
charge No. 22. Four additional charges were made, Nos. 23, 
24, 25, and 26, all similar to No. 22, with the exception that the 
quantity of borax glass was increased to 15 grammes in each 
charge. These charges worked satisfactorily in the furnace, 
but the results, which are given in Table IV, were not all that 
was hoped for. 

The data given in Table IV show that the results for gold 
and silver averaged much lower than the quantities obtained 
in the scorification assays. The quantity of silver obtained 


* School of Mines Quarterly, January, 1901, p. 157. 




TABLE III—DATA OF ASSAYS OF ZINC-BOX RESIDUES BY THE COMBINATION WET AND DRY METHOD. 

Mr. Hollis. 


ASSAY OF ORES FOR GOLD, 


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174 


NOTES ON ASSAYING. 


was very much lower, a result which may be due to the large 
quantity of litharge used in the charge, or to the quantity of 
copper present in the sample. The slag also from the crucible 
assay is richer in both silver and gold, and the second slags and 
second cupels should have been assayed—an omission which 
is to be regretted. 

In order to verify the results obtained in the scorification 
method by Mr. Hollis, a duplicate set of experiments were made 
by Mr. Kehew on residues from the same lot of samples. Mr. 
Kehew conducted the assays in a muffle that was fired by gas, 
and measured the temperature of the experiment tests by a Le 
Chatelier pyrometer. 

The same care was observed in taking the samples, and the 
same button-balance was used to weigh the samples, although 
a different set of assay weights was used. 

Three charges were made, as follows: 0.05 A.T. of residues 
was mixed with 35 grammes of test-lead in a 3-in. scorifier; on 
this were placed 30 grammes of test-lead and a final cover of 10 
grammes of borax glass. The charges were placed in a very 
hot muffle, which was of the dimensions 12 in. by 6.25 in. by 4 in., 
and the door was closed for 5 minutes; the door was then opened 
and the heat lowered. No spitting took place. The charges 
were run so that they were just driving, but upon pouring it was 
found that the temperature had not been sufficiently high to 
decompose all of the charge. The results were therefore rejected. 
Three similar charges were then made, Nos. 27, 28, and 29, 
and placed in the muffle in the position shown in Fig. 6. 
The test was conducted as before, but at a higher 
temperature (780° C. by pyrometric measurement). 
The resultant lead buttons were too large for cupella- 
tion and they were rescorified, with the addition of 2 
'grammes of silica. The second lead button was cupelled, weighed, 
Tecupelled, with the addition of C.P. silver, and parted with 
rthree strengths of nitric acid, having specific gravities, respectively, 
of 1.16, 1.20, and 1.28. The results obtained are given in 
Table V. 

By noting the position of these charges in the muffle, it is 


Fig. 6. 
27 

28 

_^9 

Front. 



TABLE V—DATA OF SCORIFICATION ASSAYS OF ZINC-BOX RESIDUES (DUPLICATES OF TABLE I). 

Mr. Kehew. 


ASSAY OF ORES FOR GOLD, 


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NOTES ON ASSAYING. 


176 

seen that the quantity of gold found in the slag increased with 
the increased temperature, i.e., it was greatest in the back of 
the muffle and least in the front. 

Owing to the fact that these assays as conducted in a 3- or 
3.5-in. scorifier yielded a button of lead which w T as too large to 
cupel, the subsequent assays were allowed to become covered 
over with the slag, which was then poured as much as possible; 
the scorifier was then replaced in the muffle and the scorification 
continued until the dead eye’ was of a diameter of 0.5 in.; the 
content of the scorifier was then poured. 

Charges Nos. 30, 31, 32, and 33 were of the same composition 
as Nos. 27, 28, and 29, and were placed in the muffle in the po¬ 
sition shown in Fig. 7. The slags from these assays 
were ground, passed through a 40-mesh sieve and 
assayed. The buttons from two of the assays passed 
into the cupel and were lost; the other two were 
weighed and parted. The data given in Table V 
show that more gold was recovered from the slag in 
tests Nos. 31 and 33, which were in the back of the muffle, than 
in Nos. 30 and 32, which were in the front. The temperature 
in the back of the muffle was 780° C., while in the front it was 
720° C. 

Charges Nos. 34 and 35 consisted of 0.05 A.T. of residues, 
mixed with 6 grammes of litharge in a 3-in. scorifier, having 
placed on top 40 grammes of test-lead, followed with a cover of 
10 grammes of borax glass. The charges were placed in a very 
hot muffle and the door closed for 10 minutes, after which it 
was opened and the temperature allowed to fall to 780° C. As 
soon as the buttons had become covered, the slag was poured 
from them, but the buttons finally obtained were too large for 
cupellation and had to be rescorified. All slags and cupels were 
assayed as usual. Although charge No. 34 was fairly satisfactory, 
for some unaccountable reason the results for charge No. 35 were 
too low. 

Charges Nos. 36 and 37 consisted of 0.05 A.T. of residues, 
mixed with 65 grammes of test-lead with a cover of 10 grammes 
of borax glass, and charges Nos. 38 and 39 consisted of 0.05 


Pig. 7. 


33 

3 i 

32 

30 


Front. 



ASSAY OF ORES FOR GOLD , 


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178 


NOTES ON ASSAYING. 


A.T. of residues, with io grammes of litharge and 30 grammes 
of test-lead, having an additional quantity of 30 grammes of test- 
lead placed on top with a final cover of 10 grammes of borax 
glass. Charges Nos. 36 and 38 were placed at the back of the 
muffle and Nos. 37 and 39 at the front. The temperature of 
the muffle was maintained as nearly as possible at 790° C. Owing 
to' the door of the muffle having been opened too soon, charges 
Nos. 37 and 39 spit, the former quite badly. The slag from 
No. 38, which was at the back of the muffle, contained more gold 
than that from No. 39, which was in front, but the slag from 
charge No. 37, in the front, carried more gold than that of No. 36, 
which was at the back. 

Considerable difficulty was encountered in parting the but¬ 
tons when the ratio of silver to gold was 2.5 to 1, even after they 
were annealed and rolled thin. No difficulty resulted, how¬ 
ever, when the ratio was 3.5 to 1. The data pertaining to these 
assays are given in Tables V and VI. 

Taking the best results, i.e., charges Nos. 27, 28, 29, 30, 31, 
32, 33, 36, and 37, an average of 4699.2 oz. gold per ton is ob¬ 
tained; the difference between the highest and the lowest results 
being 18.4 oz., which corresponds to 0.39 per cent. Averaging 
the results for silver from these assays, the figure of 4209.7 oz. 
per ton is obtained; the difference between the highest and the 
lowest results being 29.6 oz. per ton, or 0.7 per cent. 

In those charges in which litharge was used, Nos. 34, 35, 38, 
and 39 (omitting No. 39), the average determination of the gold 
was 4697.3 oz. per ton, and of silver 4202.4 oz. per ton. 

A comparison of the final average results obtained from the 
scorification assays of the zinc-box residues, obtained by Mr. 
Hollis and Mr. Kehew, is given in Table VII. 

Mr. Kehew confirmed his results of the scorification assay by 
the wet method of Mr. C. Whitehead, as follows: 0.05 A.T. of 
the residues was placed in a 250-c.c. casserole and 50 c.c. of 
water was poured over it, followed by 25 c.c. of strong nitric acid 
(sp. gr. 1.42). The casserole was then placed on a hot-plate and 
allowed to stand for 2 hours. The residue after filtration 
should have consisted of gold and other insoluble material, but 


ASSAY OF ORES FOR GOLD. 


179 


the button obtained by cupelling this residue contained silver, 
which necessitated a second cupellation, with the addition of 
C.P. silver. The slags and cupels were assayed, the final results 
being given in Table VIII. 

TABLE VII.—COMPARISON OF RESULTS OF SCORIFICATION 

ASSAYS OF ZINC-BOX RESIDUES. 

Obtained by Mr. Hollis and Mr. Kehew. 



Gold. 

Silver. 


Ounces. 

Value at $20.67 

Ounces. 

Value at $0.50. 

Mr. Hollis *. 

4694 

$97025 

4 i 7 8 -5 

$2089 

Mr. Kehew f. 

4699.2 

97 I 33 

4209.7 

2104 

Difference. 

5 - 2 

108 

3 1 • 2 

15 

Difference percentage. . . 

0.11 


o -75 


* Using coke fuel. t Using gas fuel. 


TABLE VIII.—DATA OF ASSAYS OF ZINC-BOX RESIDUES BY THE 
WHITEHEAD WET METHOD. Mr. Kehew. 


Number of 
Charge. 

Gold. 

Silver. 

Total Content. 

Ounces per Ton. 

Total Content. 

Ounces per Ton. 

40 

41 

0.23468 

0.23496 

4693 • 6 
4699.2 

0.20852 

0.20919 

4 T 7°-4 

4183.8 


The results for gold as shown in Table VIII are not as high 
as those of many of the scorification assays; the results for silver 
are quite a little lower, and confirm the results obtained by Mr. 
Hollis. 

From the foregoing experiments the following conclusions 
may be drawn: 

1. That the zinc-box residues must be in a sufficiently fine 
state of division to pass at least through a 200-mesh screen. 

2. That assay-balances, or balances of equal delicacy, must 
be used for weighing the residues. 

3. That the results obtained by the scorification assay, when 
properly made, are as accurate for the determination of the 
gold as those of any other method tried, and more accurate for 


































i8o 


NOTES ON ASSAYING . 


silver than the Whitehead wet method or the combination wet 
and dry method. 

4. That the most satisfactory charge is 0.05 A.T. of residues, 
mixed with 35 grammes of test-lead in a 3- or 4-in. scorifier, with 
30 grammes of test-lead placed on top and a final cover of 10 
grammes of borax glass. 

5. That a large quantity of borax glass is absolutely neces¬ 
sary. 

6. That the spitting of the charge can be avoided by placing 
the scorifiers in a very hot muffle, keeping the muffle door closed 
for at least 5 minutes after the charge had become fused, and then 
opening the door and reducing the temperature to from 780° to 
8oo° C. 

7. That the addition of a small quantity of litharge to a charge 
seems to lessen the danger of spitting, but when so added the 
results for silver will probably be low.” 

ASSAY OF COPPER MATTE, COPPER BARS, OR COPPER FOR GOLD. 

Take four, eight, or twelve portions of A.T. or of t l A.T. 
each, place in 3" or 3J" scorifiers, and proceed through the 
first scorification as in Method II under Silver Assay, page 50. 
Then, instead of placing the lead button resulting from each 
scorification in a separate scorifier, place four buttons in one 
scorifier, add sufficient lead to bring total weight of buttons and 
lead up to 75 or too grammes, add 1-2 grammes of fine Si 0 2 and 
2 grammes borax glass, and proceed as before. Continue scorifi¬ 
cation until lead button is fit to cupel. 

If there is not sufficient silver present for parting, add some 
and cupel the lead button as usual. 

Combining four lead buttons in this way seems to give more 
satisfactory and higher results than if the four lead buttons are 
carried through separately and the four silver and gold beads 
parted together. 

Some claim that the gold absorbed by the cupel is very much 
less where the amount of silver added is large than it is when 
just two and one-half times the amount is present or is added. 


ASSAY OF ORES FOR GOLD. 


181 


Combination Wet and Dry Method.—Unless special pre¬ 
cautions are taken, this method will give lower results for gold 
than the all-scorification method. 

Mr. W. R. Van Liew has shown in the Engineering and 
Mining Journal , April 28, 1900, that nitrous acid (HN 0 2 ) com¬ 
bined with HN 0 3 dissolves gold, and this more readily in a hot 
than in a cold solution. Nitric oxide (NO), nitrogen peroxide 
(N 0 2 ), and nitrogen trioxide (N 2 0 3 ) combined with nitric acid 
have no appreciable effect on gold whether the solution is hot or 
cold. 

Mr. Van Liew recommends the following method for copper, 
copper bars, etc. 

“Take two samples of 1 A.T. each. Treat with 350 c.c. of 
very cold water and 100 c.c. of HN 0 3 (sp. gr. 1.42) in beakers 
and set aside in a cool place. The heat evolved, by the conver¬ 
sion of NO to N 2 0 3 and N 0 2 , is considerable, but owing to the 
bulk of water present the temperature is kept down to 15 0 or 16 0 C. 
At this temperature and with the degree of acid strength the 
dissolving of the copper takes place very slowly. At the end of 
18 or 20 hours enough acid is added to take in solution the rest 
of the copper; this amount will vary from nothing to 30 c.c. 
HN 0 3 (sp. gr. 1.42), depending upon the fineness of borings or 
granulations. At the end of 24 or 26 hours the solution of the 
copper is complete. Instead of removing the lower oxides of 
nitrogen by boiling or heat, air, at a pressure of 2 oz., is conducted 
through a pointed glass tube, when the space between the surface 
of the solution and the watch-glass, clear until then, becomes 
filled with the reddish-brown fumes of N 2 0 3 and N 0 2 . At the 
end of 20 or 30 minutes these lower oxides of nitrogen are entirely 
removed. Any form of hand-blower can be substituted for the 
production of air in place of compressed air. 

“ By this method of solution heat is applied at no stage of the 
process, and the loss of gold is thus minimized. 

“ Experiments having shown that no difference was made, 
whether the gold was filtered off before or after the addition of 
the normal NaCl solution, there was added to the cold solution 
of Cu(N 0 3 ) 2 an excess of from 2 to 4 c.c. of normal NaCl solution, 


NOTES ON ASSAYING . 


182 

besides that amount necessary to precipitate all the silver present. 

“ The next morning the AgCl is filtered off, the entire contents 
of the filter washed to the point of the filter-paper, and the mass 
of AgCl covered with 4 to 6 grammes of test-lead. The drained 
papers are then placed in 2\" scorifiers, whose bottoms contain 
about a gramme of test-lead; the papers are then»dried and burned 
in a furnace not yet to a temperature of incipient redness, the 
filter-papers not being allowed to burn to a complete ash in the 
furnace, but, removed at the end of the yellow flame of the papers* 
have, when they attain a heat sufficiently great to burn the carbon 
of the charred paper outside of the furnace, this slow combustion 
taking place at a temperature too low to cause any loss of Ag in 
being reduced from AgCl. At the end of 20 minutes the papers 
will have ashed, when no more lead is added, but from 3 to 4 
grammes of PbO and 3 to 4 grammes of borax glass. 

“The copper all having been washed away in transferring the 
AgCl to the point of the filter, no scorification is necessary to get 
rid of any impurities, so this operation is merely one of melting 
and collecting the Ag and Au, the scorifier being poured as soon 
as the slag is hot enough. The resulting buttons of lead weigh 
from 4 to 5 grammes, and are cupelled at a temperature giving 
heavy litharge feathers, and allowed to blick at the temperature 
they are run, there being no difficulty as to every trace of lead 
going off when no other impurities are present. When run this 
way, duplicates easily check on the silvers within 0.2 and 0.3 oz. 
The time of operation is 48 hours, instead of 24 hours by the usual 
method.” 

Results by this method on rich material should agree within 
0.02 or 0.03 of an ounce. 

Determination of Gold and Silver in Star Antimony.*— 

“ First Method .—Pulverize fine. Take 500 grains and mix with 
3000 grains of litharge. Transfer to a small earthen crucible and 
heat at a red heat until the contents are tranquil, which will 
be about 15 minutes. The crucible is partly covered during the 
operation. 

* See Journal of Society of Chem. Industry, vol. 12, April 29, 1893, by E. A. 
Smith. 




ASSAY OF ORES FOR GOLD. 


183 


Pour, and when cold separate the button from the slag, scorify 
and cupel. Weigh the button and part in the usual manner. 
The slag may be cleaned by re-fusing it with some litharge and 
charcoal. 

Second Method .—Take antimony, 500 grains; litharge, 
1000 grains; nitre, 200 grains; carbonate of soda, 200 grains. 
Fuse at a dull red heat until quiet, the crucible being partly 
covered. Time of fusion about 15 minutes. The buttons of lead, 
which should be perfectly malleable and weigh about 500 grains, 
can be cupelled directly. The slags should be cleaned as in 
first method. The results are very satisfactory, and apparently 
more so than in the first method.” 

Determination of Gold and Silver in Metallic Bismuth.—Take 
J A.T. or 1 A.T. of the metal in the condition of borings or of 
chippings and cupel directly. Bismuth cupels as readily as lead, 
but the temperature should be kept lower. 

Weigh the resulting bead and part in the usual manner. 

One authority claims that the bismuth can be recovered from 
the cupels as follows: 

Break off and discard all that portion of the cupel not stained 
by the oxide. Grind through an 80-mesh sieve. Fuse in a cru¬ 
cible with the following charge: 


Mix thoroughly, after 
reserving a small 
amount of soda and 
borax to place on 
top. 


Cupel and bismuth oxide. 1 A.T. 

Fluorspar. 24 grammes 

Na 2 C 0 3 . 12 

Borax. 6 

Charcoal. 1 “ 


Fuse as usual, pour, and weigh the resulting button of metallic 
bismuth. 

Assaying Solutions containing Gold.—These solutions are 
generally either cyanide or chloride of gold. Upon them I have 
tried the following methods: 

1. Solution evaporated with 40 grammes of litharge. 

2. “ “ “ 3 to 5 grammes of silica and 40 

grammes of litharge. 

3. Solution evaporated with 3 to 5 grammes of silica, 5 







184 


NOTES ON ASSAYING. 


grammes of borax glass, or 10 grammes borax and 40 grammes 
litharge. 

4. Solution evaporated with J gramme soap and litharge and 
silica. 

5. Solution evaporated with 1 gramme Si 0 2 and 1 gramme 
of either coarse or fine charcoal. 

6. Solution evaporated to a very small bulk and added to the 
charge already weighed out in a glazed crucible. 

7. Solution evaporated in a lead-tray. 

The amount of solution taken may be 2 A.T. to 500 or 1000 c.c., 
depending upon the amount of gold in the solution. In regard to 
the methods, I would say that I have given them to different 
students, and some find one method most satisfactory and some 
another. . 

Methods 1 , 2, and 3. —A given amount of the solution is evap¬ 
orated in a casserole or evaporating-dish over a steam-bath to 
such a small bulk that when the Si0 2 , litharge , and other reagents 
are added , they absorb praciically all the liquid. The contents are 
stirred and heated until dry. The object is to keep the material 
granular and prevent its sticking to the dish. When dry it is fused 
with some soda and a reducing agent in a crucible which has 
been previously used, or one glazed with borax, and the result¬ 
ing lead button, weighing 26 grammes, is cupelled. 

If any of the residue sticks to the dish, it may be removed by 
rubbing it with a little fine silica or glass, which is then added to 
the rest of the charge in the crucible. 

Method 4 is conducted in the same manner as 1, 2, and 3. 
The soap is added to prevent spattering. 

Method 5.—This seems to work especially well upon solutions 
of AuC 1 3 , for the Au is precipitated in a metallic state upon the 
. charcoal. Evaporate solution as in methods 1, 2, and 3. The 
residue of charcoal and silica is then mixed with soda and 
litharge, which have been weighed out in a glazed crucible and 
the charge fused. The little particles of gold, in this case, are 
in direct contact with the carbon, as it reduces lead and it makes 
a most satisfactory fusion. Coarse charcoal seems preferable 
to fine. 

Method 6. —This is the least reliable, because, unless the cru- 


ASSAY OF ORES FOR GOLD. 185 

cible is well glazed and shows no cracks, some absorption by the 
crucible will take place. 

Method 7. —If the solution contains only salts of gold and is 
not acid, this method seems preferable to any of the preceding, 
because the lead tray and its contents can be cupelled directly. 
If the solution is acid, it will of course corrode the tray, and if 
many salts, like CaS 0 4 are present, the tray and contents can¬ 
not be cupelled, but will have to be scorified, which of course is 
not desirable in an assay for gold. Test the lead-tray with water 
to see if it is tight before adding the solution. Towards the end, 
when little solution is left, the lamp should be turned out. 

In any of the methods where the solution is evaporated and 
the residue transferred to a crucible, the most important thing 
is to see that this residue does not stick hard to the vessel in 
which the evaporation has taken place. A slight adhesion does 
no harm, for this can easily be removed by rubbing the vessel 
with a little fine silica. 

If the solution is very poor in gold, it will be necessary to add 
C.P. Ag when cupelling and then part the resulting button. If 
the solution carries both Au and Ag, a separation will of course 
have to be made. 

Some results obtained are as follows: 


Lot A. 

Lot B. 

Run VI. 

Lot C. 

Method. 

Gold in 

150 c.c. 
KCy, AuCy. 
Grammes. 

Method. 

Gold in 

200 c.c. 
Grammes. 

Method. 

Gold in 

200 c.c. 
Grammes. 

Method. 

Gold in 

200 c.c. 
AuCl 3 . 
Grammes. 

I 

.00115 

I 

.00296 

2 

.00474 

1 

.01089 

2 

.00117 

2 

.00290 

2 

.00470 

1 

.01088 

6 

.00114 

6 

.00298 

2 

.00478 

1 

.01090 





2 

.00474 




The results in Lot C were checked by precipitating the 
gold from 200 c.c. of solution with FeS 0 4 , which gave .01087 
grammes as an average of four determinations, and also by 
HoS, which gave .01084 grammes as an average of three determin¬ 
ations. 

The gold in a solution of AuC 1 3 can also be determined by 
























NOTES ON ASSAYING . 


186 


throwing it down by means of H 2 S, FeS 0 4 , oxalic acid, or alumin¬ 
ium-foil. 

Filter on a small filter, and while the filter-paper and contents 
are still moist wrap in C.P. sheet lead and cupel with or without 
the addition of C.P. Ag, as it seems best. 

The cupelling is most successfully done by having a cupel very 
hot, bringing it out to the mouth of the muffle and then dropping 
in the C.P. lead and contents. Allow the filter to burn slowly, 
and gradually push the cupel back into the muffle until the lead 
begins to drive. If any little pieces of lead appear on the sides 
of the cupel, tip the cupel and collect them. 

Electrolytic Deposition. —This is probably the most accurate 
of any of the methods, and either solutions of AuCN,KCN cr 
AuC 1 3 can be valued by it. 

AuCN,KCN Solution .—Take the same amount of solution as 
in the other methods and evaporate to ioo or 125 c.c. For elec¬ 
trodes take thin pieces of C.P. lead-foil and attach to platinum 
wires. Keep the Pt wires out of the solution. 

Use about .05 to .12 amperes. Allow to run overnight. 

The lead cathode, with deposited metals, is then detached, 
placed on a piece of C.P. sheet lead, wrapped up tightly, and 
cupelled directly. 

The following are some comparative results: 


Electrolytic. 

Evaporation with PbO and 
silica. 


Solution No. 2. 

AuCN.KCN 
(150 c.c.). 

Gold .054 oz. Silver 


“ .058 oz. 


Solution No. 15. 
AuCN.KCN 
(150 c.c.). 

.381 oz. Gold 1.35 oz. 


.385 oz. “ 1.35 oz. 


Chiddey’s Method. — Mr. Alfred Chiddey* suggests the fol¬ 
lowing: “Introduce into a porcelain dish 4 A.T., or mo'e, 
of the solutions to be assayed, add 10 c.c. of a 10 per cent 
solution of acetate of lead, then 4 grammes of zinc shavings; 
boil a minute, add 20 c.c. of HC 1 . When the action has ceased, 
boil again; wash the spongy lead with distilled water; transfer 
it with a stirring-rod to a piece of filter-paper; squeeze into a 
compact lump and place in a hot cupel. 


* Eng. and M. J., March 28, 1903, p. 473. 






ASSAY OF ORES FOR GOLD. 


187 


“The mouth of the muffle should contain a piece of dry pine 
wood, so that the muffle is filled with flame at the moment of 
introducing the spongy lead. 

“ In the case of very dilute nearly pure gold solutions I would 
suggest the addition of a known quantity of nitrate of silver dis¬ 
solved in cyanide before adding the acetate of lead.” 

The following are some comparative results: 



Evaporation with 
Litharge, Soda, 
and Charcoal. 

Evaporation in a 
Lead Tray. 

Chiddey’s Method. 

150 cc. of a solution of 
AuCN.KCN. 

Gold .590Z. 

Gold 1.23 “ 


Gold .59 oz. 
Gold 2.01 “ 
Silver 3.98 “ 
Gold .21 “ 
Gold 1 . 17 “ 

100 cc. of AuCN,KCN. . . j 

100 cc. of AuCN,KCN. . .. 
50 cc. of AuC1 3 . 

Gold 1.98 OZ. 
Silver 4.06 “ 
Gold .20“ 




My experience with this method is that, as a rule, it is a 
most excellent one and by far the shortest of any suggested up 
to this time. The following additional data may prove helpful. 
Always roll the zinc shavings lightly into a ball between the 
hands so as to have them compact with few ends protruding. 
Care must be taken to have sufficient lead acetate present; 15 to 
20 c.c. are none too much for a large amount of solution or for 
one rich in precious metals. Have sufficient HC 1 (sp. gr. 1.2) 
present to dissolve the zinc, but not much in excess. The lead 
thrown down should enclose the zinc completely, otherwise the 
latter will break up into fine threads, rendering it necessary to 
filter. The presence of copper in the solution seems harmful, 
causing the zinc and lead to break up and become finely divided. 
In many solutions the spongy lead thrown down, when placed 
on the cupel, is not sufficient to collect the precious metals into 
one bead, so I prefer to wrap it in lead-foil and then cupel. On 
gold chloride solutions the method seems to give low results. 
Heating this solution decomposes it, throwing down metallic gold, 
which makes filtering necessary, so the solution must be kept 
cold. 














NOTES ON ASSAYING. > 


3 88 

The following are some other methods which have been 
suggested for valuing cyanide solutions for gold: 

“Take 15 A.T. or more of solution, add 15 to 25 c.c. of strong 
H2SO4 and 6 to 12 grammes of zinc-dust. Warm and stir at 
intervals for at least 10 to 12 hours. Dissolve any zinc remaining 
and then filter. Burn the filter in a crucible containing part of 
the flux for the crucible-charge, cover with the remaining flux 
and some borax glass, fuse and assay. Sufficient time must 
be allowed in this method in order to be sure that all the gold 
has been precipitated.” 

Miller’s Method. E. & M. J., July 23d, 1904, p. 997. 

“Take 1000 c.c. of solution and put in a 2-litre flask. Add 
1 to 2 grammes of powdered copper sulphate. Agitate. Add 
10 to 15 c.c. of concentrated HC 1 and agitate thoroughly. Filter, 
dry the precipitate on the filter, burn and assay the precipitate 
either in a crucible or scorifier, preferably the former.” 

Lindeman’s Method. E. & M. J., July 7th, 1904, p. 5. 

“Ten A.T. of solution are heated until quite hot, ammoniacal 
copper nitrate is then added until the solution shows a permanent 
blue color. Sulphuric acid is then carefully added in excess, 
the solution stirred and immediately filtered. The paper is 
folded and carbonized in a scorifier, transferred to a crucible, 
fused and cupelled.” 

Arent’s Method. A. I. M. E. Albany meeting, Feby. 1903. 

“Take 250 c.c. of the solution to be tested; add a few c.c. of 
H2SO4; agitate for several seconds and then add not less (al¬ 
though not much more) than one gramme of ccment-copper. 
Heat to boiling. This is kept up for about 10 minutes, so that 
the rising steam-bubbles keep the mixture well agitated. The 
mixture is then filtered through a 7-inch-diameter gray filter- 
paper. No washing is done. As soon as the filtering is finished, 
one-third of a crucible-charge of flux is added to the filter con¬ 
taining all the sediment of the mixture. Some of the moisture 
is rapidly absorbed by the flux, which permits the folding of 




ASSAY OF ORES FOR GOLD. 


189 


the filter’s rim upon the charge and its subsequent removal 
without loss or tearing. One-third of a crucible-charge of 
flux having previously been placed upon the bottom of the 
crucible which is to be used for melting, the filter is trans¬ 
ferred to the crucible, well tucked down, and the last one-third 
of the crucible-charge is placed on top of the filter in the crucible. 
It is then ready for the furnace. The filter furnishes the reduc- 
ing-agent for the assay.” 

“Use 30 grammes litharge and the usual amount of borax: 
and soda, employing an F crucible for melting. About 20 
grammes of lead are obtained which, upon cupelling, furnishes 
a bead free from copper.” 

The use of precipitants other than copper and copper salts 
seems to me advisable, because with copper we are making use 
of a metal known to be deleterious in the cupellation process and 
one which we especially endeavor to eliminate beforehand. 




CHAPTER V. 


ASSAY OF ORES FOR LEAD. 

Lead fuses at 327 0 C., sp. gr. = 11.35, atomic weight = 206.95. 

The principal ores of lead are: 

Galena, PbS (sp. gr. 7.4 to 7.6), with 86.6% lead when pure. 

Cerussite, PbC 0 3 (sp. gr. 6.46 to 6.57), with 77i%*lead when 
pure. 

Anglesite, PbS 0 4 (sp. gr. 6.12 to 6.39), with 68.3% lead when 
pure. 

Pyromorphite, 3Pb0,P 2 0 5 +PbCl 2 (sp. gr. 6.5 to 7.1), with 
76.3% lead when pure. 

Besides these we have many complex compounds, such as 
Bournonite, PbS + Cu 2 S-f Sb 2 S 3 (sp. gr. 5.7 to 5.9). 
Jamesonite, 2PbS+Sb 2 S 3 (“ “ 5.5 to 6). 

While the assayer may be given ores or concentrates similar 
to the first four, he usually has submitted to him ores or products 
of a much more complex and a much baser character, such as 
PbS + FeS 2 +PbC 0 3 in a quartz or silicious gangue; 
PbS+ZnS+FeS 2 “ “ calcareous or silicious gangue; 
PbS+PbC 0 3 “ “ calcareous gangue. 

He may also have submitted to him furnace products, litharge 
(92.86% lead when pure, sp. gr. 9.2 to 9.36), old cupels, slags, 
etc. Although the fire assay for lead is less accurate than the 
wet method, still it is in general use at smelting works for assay¬ 
ing ores and furnace products, because lead ores are always bought 
and sold on this assay and not upon the wet analysis. 

The reasons for its inaccuracy are the following: 

1 st. Because lead and lead sulphide are both volatile at 
moderate temperatures. Results low. 


ASSAY OF ORES FOR LEAD . 


191 

2d. Because impurities of various kinds (Cu,Sb) are reduced 
with the lead and pass into the button. Results high. 

3d. Because of the tendency that lead and its compounds 
have to slag, which tendency is increased by the presence of 
arsenic, antimony, and zinc. Results low. 

For these reasons we must be especially careful of the heat, and 
the slag must be made as simple as possible, easily fusible, and 
must not be too acid. 

For convenience, lead ores may be divided into— 

1st. Those which contain sulphur. 

2d. Those which do not contain sulphur. 

Our object, in either class, is to flux the gangue of the ore 
and reduce the lead present, jrom whatever combination it may 
be in, to metallic lead; therefore the simpler we are able to make 
the fluxes the lower we may keep the heat, and the shorter the 
time of fusion the better it will be. Keep, however, in mind 
that the ore must be decomposed and the slag perfectly liquid. 

The ores should, like those for the assay of silver and gold, 
be fine enough to pass through a 100-mesh sieve and should have 
been dried at ioo° C. 

Make all fusions in a muffle-furnace, unless specified otherwise , 
and report the results in percentage to first place of decimals. 

The assay consists of a reducing fusion with iron and some 
reducing agent, such as argols, charcoal, flour, etc. The reducing 
agents take away the oxygen from the PbO present or formed 
during the fusion, and the iron removes the sulphur: 

2 PbO + C = 2Pb+ C 0 2 ; PbS+ Fe = FeS+ Pb. 

Nails, iron, or iron crucibles are absolutely necessary, whether 
there is sulphur present in the ore or not, for three reasons: 

1. 4 K 2 C 0 3 + 7PbS = 4 Pb+ 3(K 2 S,PbS) + K 2 S 0 4 + 4 C 0 2 . 

By adding iron we obtain 

(K 2 S,PbS) + Fe = (K 2 S,FeS) + Pb. 

That is, if iron was not present, the double sulphide of lead 
and the alkali, used as a flux, would pass into the slag giving 
low results. 



192 


NOTES ON ASSAYING . 


2. PbS+Fe = FeS+Pb. 

3. 2Pb0,Si0 2 +2Fe = 2Fe0,Si0 2 +2Pb. 

The siliceous impurities are generally quartz, feldspar, and 
complex silicates; the basic ones are limestone and oxide of iron. 
The gangue may also be barite and sometimes fluorspar. 

The fluxes employed are sodium or potassium carbonate for 
the silica, borax glass for the oxides and limestone, fluorspar for 
barite, argols or charcoal for a reducing agent, and iron as a 
desulphurizer. 

In this assay, as in the assay of ores for silver and gold, we 
find the following: 

Silver and gold in the ore both pass into the lead button. 

Copper goes partly into the lead and partly into the slag; a 
high temperature and a great excess of reducing agent will tend 
to make it pass into the lead. 

Zinc partly volatilizes, partly slags, and a small quantity passes 
into the lead. 

Antimony goes partly into the slag, but most of it into the lead. 

Arsenic (depending upon the temperature of the fusion) either 
forms a speiss with the iron present or else partly slags and partly 
volatilizes. (See page 137.) This speiss is lighter than the lead 
and will be found as a round brittle button on top of or embedded 
in the lead. From the foregoing data it will be seen that if our 
ore is very impure, that is, if it contains much Cu, Sb, or As, our 
results will be rather unsatisfactory. In ores of this character we 
have to resort to the wet analysis. 

Sulphide Ores. (Fusion in the muffle.)—Sulphate of lead, 
either natural (anglesite) or obtained from chemical works, is 
treated in the same manner. 

It is recommended to use at least 10 grammes of the material 
to be analyzed and if possible 20, for my experience is that 
io'grammes give better results than 5, and 20 oftentimes better 
than 10. 

The following proportions of fluxes to ore will serve as an 
example for our assays. Refractory sulphides, like sulphide of 
zinc and those with a basic gangue, will probably require an addi¬ 
tional amount of borax glass. 


ASSAY OF ORES FOR LEAD. 


193 


NaHCOg.20-40 or 


u 


u 


u 


u 


u 


a 


Mix 
in the 
crucible. 


Ore. . 10-20 grammes * ] 

' NaHC 0 3 
1 K 2 C 0 3 . . 

Borax glass. 3-5 

Argols. 5 

Nails (tenpenny). 

or one rail-spike (2J" or 3" long) 5 
Cover of salt ij" deep if possible. 

The soda and potash, besides acting as fluxes, probably act in 
this way: K 2 C 0 3 =K 2 0 + C 0 2 ; PbS + K 2 0 + C = K 2 S + CO + Pb. 

If the borax glass is used in too large quantity, it will make the 
slag too acid, and the lead will pass into it. In a clean galena 
we shall need only a gramme or two, while in limestone carrying 
PbS or in an ore carrying much ZnS we shall require as much as 
6 or 8 grammes. In place of nails as a desulphurizer, some assay- 
ers use coils of coarse wire or a strip of broad iron bent on the 
curve of the crucible. Using 5 grammes of ore has always given 
me lower results than when using 10 grammes. If, however, 5 
grammes are used, diminish the soda and borax glass a little, but 
keep the argols and the nails the same. 

Fusion in the Muffle.—Place the crucible, by means of a pair 
of tongs, in a good hot muffle and close the door of the muffle 
with a plate or with some charcoal or with both. When the con¬ 
tents of the crucible begin to fuse, which is indicated 
by small jets of flame leaping up from the interior of 
the crucible, close the draft of the furnace and lower 
the temperature, to prevent the contents of the cru¬ 
cible from boiling over. When the danger of this is 
over, keep the heat at scorifying temperature or lower 
for 20 to 25 minutes, then raise the temperature for 15 
to 30 minutes more to the greatest temperature of the 
muffle. The whole period of fusion should be from 35 
to 55 minutes, depending upon purity of the ore and 
the character of its gangue. Take the crucible from the furnace, 
but do not set it down, catch the nails with a pair of small hand-tongs, 
tapping them gently while holding them in the slag, then remove. 


c 


> 


w 


* With 10 grammes of ore use A, and with 20 grammes of ore use B crucible. 















■194 


NOTES ON ASSAYING. 


(If drops of lead are on the m ils, return crucible to muffle, for the 
fusion is incomplete.) Pour the fusion into a mould and when cold 
separate the lead from the slag and hammer clean. Weigh to 
second place oj decimals and report the result in percentage. Dupli¬ 
cate assays should agree within one half of one per cent, and a 
pure galena should give 83!% to 84% of lead. 

The following fluxes may be used in place of soda and argols: 

White Flux , made by deflagrating together equal parts of 
saltpetre and argols. 

Black Flux , made by deflagrating together one part of saltpetre 
with two or three parts of argols. 

Black Flux Substitute is a mixture of ten parts of carbonate of 
soda and one to three parts of flour. 

Cyanide of Potash Method. (Fusion in the muffle.)—This 
method has one great advantage over any of the preceding ones, 
in that it can be done at a much lower temperature. Still it 
seems to give lower results than the ordinary muffle assay or the 
iron-crucible as ay, which may be due to the KCN holding some 
lead sulphide in solubon The charge is made up as follows’ 

Ore, 10 grammes; KCN 3 , 25 grammes. Mix. ( The KCN is 
a deadly poison 1 ) Fuse for 25 minutes at a low temperature and 
look out jor jumes: 

PbO + KCN = Pb+KCNO, also PbS + KCN =Pb + KSCN. 

Sulphide Ores. (Fusion in pot-furnace.)—These ores may 
be assayed in this way, using the same or larger quantities of ore 
and fluxes as were used in the muffle assay. The method is much 
more difficult, however, and great care has to be exercised in the 
heat, otherwise the loss by volatilization will be very great. 

When we have a rich or fairly pure ore we can make an assay 
O in an iron crucible to great advantage. The crucible, which 
should be of very heavy wrought iron, takes the place of the 
nails and acts as the desulphurizer. 

Slags, Furnace Products, or Ores Very Poor in Lead. (Fusion 
in pot-furnace.)—These can be assayed to advantage in the pot- 
furnace, for we can use a large quantity of slag or of ore. Use E 
or F crucible. 


ASSAY OF ORES FOR LEAD. 


195 


The following charges I have found to work very satisfactorily: 


c ( 


< < 


< < 


Slag. 30 grammes 

Bicarb, soda. 60 

Borax glass. o to 5 

Argols. 6 

Nails (2openny).. 1 

or Spike. 1 

Cover of salt i£" thick. 


Mix 


Ore. 30 grammes 

(Limestone carrying I to 2 % of PbS) 

Bicarb, soda. 60 grammes 

Borax glass. 15 “ 

Argols. 8 

Nails (2openny). 2 

or Spike. 1 

Cover of salt i£" thick. 


Keep in the furnace 25 to 35 minutes after the fusion has taken 
place. Results by the above fusions, on low-grade ores or slags 
running from 2 to 5 per cent, check very closely (.2% to .3%). 
If the ore is so poor that no lead button is obtained, 200 or more 
grammes of it can be taken, panned, and the concentrates 
assayed. From this assay figure the amount of lead in the 
original ore. 

The method of adding a known amount of silver to the slag 
to be assayed and then subtracting this from the lead button 
obtained I have found inaccurate and unsatisfactory, also the 
method of adding a given quantity of PbO and allowing for the 
lead contained therein. 


Sulphide Ores in Iron Crucible. 

Ores should be quite pure. 

Ore. 5 ° 


(Fusion in pot-furnace.)— 


grammes 


Place in the 
• bottom of the 
crucible. 


Potash (K 2 C 0 3 ). S °~75 

Borax glass. 8-10 

Flour. 8 “ 

Cover of salt ij" thick. 


Mix and 
• place upon 
the ore. 


If 25 grammes of ore are taken, use only 5 to 6 grammes of 
borax glass, 30 of potash, and 5 of flour. 

Fuse for about 12 minutes or until fairly quiet, that is, until 
foaming ceases. Take the crucible from the furnace, let it cool 
a minute or so, and then pour as usual. 

Class II. Ores Containing no Sulphides. (Fusion in the 
mu ffle.)—Although these ores may be assayed in the pot-furnace, 
the student is recommended to use a Battersea A or B crucible, 





















196 


NOTES ON ASSAYING. 


or one similar in size, and make the assay in the muffle. Conduct 
the assay as in Class I (muffle fusion). Mix the ore with the 
fluxes, but never have the crucible more than two thirds full. 
For substances given below use the following: 



No. 1. 

Lead 

Carbon¬ 

ate. 

No. 2. 
Lead 
and 
Copper 
Carbon¬ 
ate. 

No. 3. 

Cupels. 

No. 4. 

Lead 

Phos¬ 

phate. 

No. 5. 

Lead 

Silicate. 

Ore, grammes. . 

10-20 

10-20 

10 or 10-20 

10-20 

10-20 

Bicarb, soda, “ 

15-15 

10 

20 

10 

3 ° 

20 

Potash, “ 

5-10 

15 

— 10 

— 

— 

Borax glass, “ 

3 ~ 3 

5 

10 

10 

5 

2 

Argols, “ 

7 ~ 7 

4 

8 

8 

5 

5 

Sulphur, “ 

— — 

1 

— — 

— 

— 

Nails (tenpenny), or one 
rail-spike 2 $— ?!' long 

3 ~ 3 


3 

3 

2 

3 


Cover of of salt in each case. 

Ores containing Mn 0 2 or Fe 2 0 3 require an additional quan¬ 
tity of argols. A mixture of carbonate of soda and carbonate of 
potash is more fusible than either one alone. 

In the phosphate ore, phosphate of soda and PbO form; 
the argols then act and, as in all the other cases, reduce the 
PbO to Pb. Iron is used in all the charges except in No. 2. It 
is used in the silicate because lead is not easily reduced from sili¬ 
cate of lead except in the presence of iron. Sulphur partly de¬ 
composes the singulo-silicate, and carbon reduces some of the lead 
from a bisilicate, but in order to extract all the lead it must be 
set free by a basic flux, and this is the reason that metallic iron 
sets free all the lead from all fusible lead silicates: 

2 Pb0,Si0 2 + 2Fe = 2Fe0,Si0 2 + 2Pb. 

This is probably the reason why in the assay of cupels and 
other non-sulphide substances we obtain higher results when iron 
is used than when it is not, for silicate of lead is either present 
or it is formed during the fusion from the ingredients of the 
crucible itself and the PbO, or from the gangue of the ore and 
the PbO. 

A bright button, separating easily from the slag, indicates too 
great a heat or too long a fusion. A bright coating between the 
button and the slag indicates too low heat or imperfect decom¬ 
position. 



















ASSAY OF ORES FOR LEAD. 


*97 


Cu, As, Sb, Zn, or S may cause buttons to be brittle. 

Cu and Sb may cause them to be hard. 

General Remarks upon the Lead Assay.—As has been previ¬ 
ously stated, it seems advisable to use as much of the substance as 
possible for the assay without filling an A or B crucible more than 
two thirds. In the West they formerly used 5 grammes, but the 
writer finds that the results are lower with 5 grammes than when 
10 or 20 are used. Where a large amount of work is being done 
the fluxes are mixed together in proportions somewhat as follows: 
5 parts carbonate of soda, 7 parts carbonate of potash, 2 parts flour, 
^ part borax glass. This mixture is kept in stock, a given quan¬ 
tity measured out and placed in the crucible; the ore is then weighed 
out, thoroughly mixed with it, and a layer of salt placed on top. 

A large amount of soda or alkali is advisable owing to reaction 
on page 67, carbonates of the alkalies throwing metallic lead 
down from a sulphide. 

In regard to the temperature used and the time given to the 
fusion, some assayers recommend a short quick fusion, others a 
long one, even up to ij hours. It seems to the writer that it is 
more a question of having the temperature just right at the 
beginning of fusion, during the first 15 or 20 minutes, when the 
main bulk of the substance is being decomposed and the lead 
compounds reduced to metallic lead, than it is a question of 
length of time of the fusion. The iron of course must not be 
covered with globules of lead when the fusion is poured, for this 
is a sure indication that it is not complete. Some do not pour 
the fusion, but allow it to cool in the crucible, which when cold 
is broken. This method will often give higher results than 
when fusion is poured. 

My reason for thinking it a question of temperature at the 
beginning is, that if six or more students are given an ore carry¬ 
ing, we will say, 82% of lead, and they are all told to use the 
same charge and fuse for 50 minutes in the muffle, some will 
obtain 81% to 82% and others only 74% to 78%; the ore, the 
charge, and the time being the same in every case, it stands to 
reason the fault must be with the temperature. 

The smelters generally pay for any lead in an ore above 
5% at so much per unit, according to a sliding scale, which varies 
from time to time. 


CHAPTER VI. 


BULLION. 

Bullion.—In the Engineering and Mining Journal of Febru¬ 
ary, 26, 1898, Messrs. C. Whitehead and T. Ulke in an article 
“The Assaying of Silver Bullion,” say: 

“Silver bullion, broadly speaking, is classified as follows: 
Dore bars are such as contain gold and base metals (chiefly 
Cu, Pb, often Sb, and sometimes S), together with from 925 to 
990 parts of silver. 

“Fine silver bars are those which are free from gold and suffi¬ 
ciently free from alloys to render them fit for coinage and for use 
in the arts. They average from 990 to 999 parts of silver per 1000. 

“Base bars contain a large percentage of base alloys, usually 
Pb, Sb, or Cu, from 100 to 925 parts of silver per 1000, and often 
gold. 

“At the United States mints and assay offices bullion con¬ 
taining less than half its weight in gold is classified as silver bul¬ 
lion, and the silver contained can not be purchased, but must be 
returned to the depositor in the shape of fine silver or merchant’s 
bars (999 fine), while both the gold and silver in bullion with 
50% and over of gold (classified as gold bullion) will be paid 
for by the Government. Except in the case of fine bars, from 
which only cuttings are taken, as in the case of gold, silver alloys 
are melted, thoroughly stirred, and are then sampled in the fol¬ 
lowing way: One or more portions, depending upon the weight 
of the deposit, are dipped from the melting-pot and poured from 
a height of 3 feet in quantities of about an ounce, in a fine stream, 
into cold water. The resulting granulations are carried in cop¬ 
per cups to the assayer’s laboratory. However, in addition it 
is advisable for the assayer to take two chips from silver bars, 
as the chip and the granulation samples check closely in gold 
and near enough in silver to identify the samples, should a 

198 


BULLION. 


r 99 


interchange of melts have occurred. After drying the granu¬ 
lations by heat, about 1-5 ounces is reserved in the assay 
room and the remainder returned. The sample lots are now 
laid out upon a board containing cup-like sockets bored at regu¬ 
lar intervals and numbered. A granulation from each sam¬ 
ple is next hammered and rolled into a thin strip, this being 
merely for convenience in cutting for the adjustment in weigh¬ 
ing the assay. Each strip is laid beside its kindred granulations 
and numbered by stamping. The board is now removed to 
the assayer’s weigh-room.” 

The silver is determined by the volumetric or Gay-Lussac 
assay, a full description of which will be found in the above 
article. 

Lead Bullion or Base Bullion.—This may be lead carrying 
either silver or gold or both silver and gold, but it is seldom as 
pure as this and generally contains Cu, As, Sb, Zn, and similar 
impurities. 

The bullion may be in bars or in the condition of borings or 
of cuttings. Samples for assay 
are taken from the bars or pigs 


C 


0000 


3 c 




by sawing, cutting, punching, or boring them. 

Dip samples, taken when the bullion is melted, or those taken 
by sawing, are the most accurate. Never cut samples off the cor¬ 
ners of a bar. Composition of the same pig varies in places. 
G. M. Roberts, in Trans. A.I.M.E., shows that samples taken 
from side of moulds or ingots may be richer than the rest of ingots. 

Preliminary Test.—To determine if the bullion can be cupelled 
directly, take 1 gramme or so (do not weigh it) and either wrap 
it in C.P. lead or drop it as it is into a hot cupel. If impurities 
like Cu, Sb, As, Sn, etc., are present in considerable quantity, 
the bullion will not drive, or else will drive and then freeze after 
a short time. 

Regular Assay.—If the bullion will not cupel, take two or 
three portions of \ A.T. each, scorify with an additional amount of 
lead (30 to 45 grammes), and then cupel. 

Bullion containing tin often gives trouble; therefore if \ A.T. 
of it will not scorify with an additional 45 grammes of lead and 
some borax glass, decrease the amount of bullion and increase 






2\00 


NOTES ON ASSAYING. 


the amount of lead and borax glass, and continue to do this until 
it scorifies satisfactorily. It is simply a question of more lead and 
borax glass, provided the heat is sufficiently high. 

If the bullion cupels, weigh out carefully two or three portions 
of J A.T. each, and be sure that each contains the same propor¬ 
tion of coarse and fine drillings as are contained in the bullion 
sample. Take two or three pieces of C.P. sheet lead which weigh 
the same, and wrap each lot of bullion in a piece of lead so that 
the whole is very close and compact. This should be done in a 
scorifier: in case the lead wrapping breaks, any bullion coming 
out will be saved. If the lump is not compact, it may overflow the 
cupel while melting, or else leave small particles on the sides of 
the cupel, whch will not come down into the main button. 

Keep the heat so low that feather crystals of PbO will always 
form. Have two or three hot cupels in the muffle to cover the 
others with as soon as the blick occurs. Have the buttons solidify 
as quickly as possible to avoid loss of Ag, and withdraw from the 
furnace slowly to avoid sprouting. 

Sprouted buttons should be rejected. 

Bullion up to 400 oz. should check within J oz. for total Ag 
and Au. 

The buttons are cleaned, hammered, or rolled out, weighed , 
and parted as described under Assay of Gold Ores. 

Report results in ounces per ton for both Ag and Au in bul¬ 
lion under 500, i.e., carrying less than 50% of both metals. 

Experiment. —Take or cut off from the bullion given to you 
about 35 grammes. If not fine or in small pieces, cut it up and 
mix thoroughly. Make the preliminary test as per page 199. 
If the button does not cupel, take two portions of ^ A.T. each 
and scorify with 40 or more grammes of granulated lead. If 
this does not scorify, use less bullion, more lead, and some borax 
glass. Then cupel as directed. 

If the bullion taken for preliminary test cupels and button 
blicks, then weigh out accurately on the pulp-balances two por¬ 
tions of \ A.T. each. 

Weigh out two portions of C. P. sheet lead from 10 to 15 
grammes each and have them balance each other. Wrap the 
bullion up very tightlv in the^e. 


BULLION. 


201 


Have the cupels hot and drop the bullion into them, and 
cupel with feather litharge crystals. When buttons blick, cover 
with hot old cupels, which have been heating in the back of 
muffle, and withdraw slowly from furnace. 

Clean and weigh the silver and gold beads. Part for gold. 

Report the following: 

1. Weight of C.P. lead used. 

2. Time of cupellation. 

3. Amount of lead oxidized per minute, including lead in bul¬ 
lion taken. 

4. Ounces per ton in gold. 

5. “ “ “ “silver. 

Silver Bullion containing no Gold.—Silver and gold bullion 
are reported, not in ounces per ton, but in fineness or parts in 
a thousand. That is, if the bar of bullion carries 95% of silver 
and 5% of gold, we say it is 950 fine in silver and 50 fine in gold. 

The best three methods for determining the silver contents are: 

1st. Fire assay. Cupellation with C.P. lead. 

2d. Volumetrically, with a standard solution of salt. 

3d. Volumetrically, with a standard solution of sulphocy- 
anide of potash (German method). 

The first and second methods require a preliminary assay to 
determine the approximate fineness of the bullion to be assayed. 

Preliminary Assay .—Calculate as per pages 203 and 204, see 
also page 205. On the fine button-balance weigh out .5000 grammes 
of the bullion, wrap it in about 5 grammes C.P. lead-foil and 
cupel carefully, keeping the heat low enough to form the litharge 
crystals, but not so low as to freeze the button. Take the button 
from the cupel, clean, hammer, and weigh it carefully in the 
usual manner. Part the button to see whether there is any gold 
in it, and then calculate the approximate fineness of the bullion. 

The whole object of the work, thus far, is to ascertain the 
approximate composition of the bullion; having done this, the 
“check assay” can be made to correspond, so that if it is found 
that there are .475 grammes of silver in .500 grammes of bullion, 
weigh out .4800 grammes of C.P. silver, wrap it up in 5 grammes 
C.P. lead-foil and cupel by the side of the regular assay. We 
weigh out .480 grammes in this case because fine bullion gener- 


202 


NOTES ON ASSAYING. 


ally loses between 4 and 5 milligrammes when .500 grammes of 
bullion are used. 

Whatever the percentage loss of silver the “check” or “proof 
assay” shows it is fair to assume that the bullion being assayed also 
sustains. 

For this reason I prefer to figure the percentage loss and 
make up the check as per the examples given on pages 203 and 
204, rather than work by the table on page 207, taken from Van 
Furman’s Manual of Practical Assaying. 

From his table the amount of C.P. silver-foil can be figured 
that it will be necessary to use in the check assay, if the work 
is being done by the first method, and the amount of bullion to 
use if the second method is employed. 

Actual Assay. —Calculate as per pages 203 and 205. 

Weigh out as accurately as possible two portions of the bullion, 
say .49900 and .49980 grammes. Wrap each just as compactly as 
possible in 5 grammes C.P. lead-foil, so the bullion will not spread 
in the cupel. Make up the proof or “ check ” as shown by the 
preliminary assay, weighing it out just as accurately as the bullion, 
and wrapping it in exactly the same amount of C.P. lead-foil as. 
was used with the two samples of bullion to be valued. 

Have three cupels heated hot, side by side in the muffle, and 
then drop the three buttons quickly into them, placing the check 
in the middle one. Close the door of the muffle, that the buttons 


Bullion Check Bullion 



may “ drive ” as soon as possible and all at once. When they are 
“driving,” open the door immediately and cupel them at as low 
a temperature as possible without freezing, always obtaining the 
feather litharge crystals; push back into muffle just before blick- 
ing. Endeavor to have them “blick” together, and then draw out 
slightly toward the front of the muffle until they chill. Just as 
soon as they have done so, cover them over with hot cupels or hot 
scorifiers, which should have been heating in the muffle; with¬ 
draw slowly from the furnace to avoid sprouting, cool, hammer. 


BULLION. 


203 


clean, weigh, and part for gold. In bullion containing gold all 
three buttons have to be parted (see page 206). 

Example No. i. —Preliminary Assay. (To determine the ap¬ 
proximate fineness of the bullion.) 

Take, for instance, *49909 gms. of bullion.. 

Silver found after cupellation, i.e., the 
bullion was wrapped in C.P. lead 

and cupelled.49144 gms., or 98.47%. 

Loss = .00765 grammes. 

The amount of silver in this loss is probably .00500 ± grammes,., 
and the remainder impurities in the bullion. 

Actual Assay. (To determine actual fineness of bullion).— 
Weigh out accurately two samples of the above bullion. 

Suppose they weigh as follows: 

Sample No. 1. Sample No. 2. 

. 49900 grammes. .49980 grammes. 

Average = .49940 grammes. 

To determine the amount of silver to weigh out for the 
check, take 


Preliminary 

assay. 

.49909 


Average of 
2 samples 
taken. 

.49940 


Ag. found in 
preliminary. 

• 49 I 44 

.OO50O i Supposed loss. 


Silver for 
check. 

x= .49674 


.49644 

The two samples of bullion and the “ check ” are wrapped in 
pieces of C.P. sheet lead, all of the same weight (six or more 
grammes), and cupelled. 

No. 1 sample . . .49900 Check (C.P. Ag). - 49 6 74 No. 2 sample.. .49980 
After cupelling. .49152 After cupelling.. . .4923° After cupelling.. .49230 

. 00444 = .89% loss of silver. 

99.11% : 100% : : .49152 silver found in sample : * (silver in sample if there 

No. 1 had been no loss of silver), 

*= .49593; - ‘ 4 ^ —= 99.38% or 993.85 fine. 

.49900 

Sample No. 2 is calculated in the same way. 








204 


NOTES ON ASSAYING . 


Example No. 2 .—Preliminary Assay .—Bullion taken = .5000 
grammes. Silver found =.2500 grammes. 

Suppose the loss of silver = .00500 ± grammes. 

Then the silver in the bullion is probably . 255 ± grammes 
and the impurities “ “ “ are “ .2451b 

Actual Assay. 

Bullion No. i. Bullion No. 2. 

.5000 grammes. .48000 grammes. 

Average = . 49000 grammes. 

Therefore .5000 : .4900 :: .2550 : #= . 2499 grammes. 

C.P. silver-foil to be weighed out for “ check ” = . 2499 grammes. 

Impurities like copper materially affect the loss of silver, and 
as we wish to have the “ check ” correspond as nearly as possible 
to the bullion, a like amount of copper is to be added to the 
“check.” This bullion should be wrapped in 15 to 20 grammes of 
C.P. lead. 

In Mitchell’s Assaying (page 628) is found the following 
table in regard to an alloy of silver and copper: 


Standard 

of 

Silver. 

Quantity of 
Copper Alloyed. 

Amount of Lead Necessary 
to Add. 

Relation of Lead 
to Copper. 

IOOO 

0 

3 /io grammes or as small 
an amount as possible 


95 ° 

50 

3 grammes 

60 to I 

900 

IOO 

7 “ 

70 to 1 

800 

200 

10 

5 ° to 1 

700 

3 °° 

12 

40 to 1 

600 

400 

14 “ 

35 to 1 

5 00 

5 00 

16 to 17 grammes 

32 to 1 

400 

600 

16 to 17 “ 

27 to I 

3 °° 

700 

16 to 17 “ 

23 to I 

200 

800 

16 to 17 “ 

20 tO I 

100 

900 

16 to 17 “ 

M 

O 

4 -* 

00 

M 


“Long experience has proved that silver opposes the oxidation 
of copper by its affinity, so that it is necessary to add a larger 
amount of lead in proportion to the quantity of silver present.” 

Sufficient lead must be added to cause the button to unite 
well. If too great an excess is used, however, the loss of silver will 
be large, owing to the duration of the cupelling process. Some 
assayers prefer to add the lead to the cupels at first, have it 











BULLION. 


205, 

driving well and then drop the sample of bullion, wrapped in 
thin C.P. sheet lead, into the lead which is already driving. 

As has been previously shown, if the cupels used in the bullion 
assays and check are saved, pulverized, and assayed by the cru¬ 
cible method, about 90% of the silver found to have been lost 
will be recovered from the cupel. This shows that the loss 
sustained during cupellation is largely due to absorption. The 
remainder of the loss can be accounted for by volatilization. 

Some experiments seem to show that 5 parts of lead are 
required to cupel an alloy of 900 Ag and 100 Cu in the middle 
of the muffle, 10 parts in the front, and 3 parts in the back; i.e., 
the higher the heat the less lead is required. 

Silver Bullion containing Gold. — Preliminary Assay. (To 
determine the approximate fineness of the bullion.)—Weigh out, 
say, .49850 grammes of bullion, wrap it in from 3 to 5 grammes 
of C.P. lead-foil and cupel. 

After cupelling, button weighs.... 49000 grammes * 

Probable loss in Ag.00500 ± 

(Gold loss is supposed to be very 
small.) 

Impurities.00350 ± “ 

.49850 ± “ 

That is to say, the silver in the bullion, originally taken, is 
probably . 48500 ± grammes. 

Actual Assay. (To determine actual fineness of bullion.)— 
Weigh out, for example, . 49860 and . 49880 grammes of bullion. 
The average = .49870 grammes. 

Then the silver in the check will be 

Prelim. Av. of two Ag probably 

bullion. samples. in prelim. 

.49850 : .49870 :: .4850 : #=.48519 

The gold will be 

.49850 : .49870 :: .0100 : #=.0100 

* Ag = .4800 = 96.28% 

Au = .0100 


.4900 grammes 








2 o6 


NOTES ON ASSAYING. 


We now have 

.Sample No. i. Check. Sample No. 2. 

.49860 C.P. Ag foil.48519 .49880 grammes 

Au from parting 

preliminary.01000 


Wrap them in three pieces of C.P. lead-foil which balance 
each other; 4 to 6 grammes will probably be enough, but it must 
be sufficient to bring the bullion all into one globule , when melted. 
After cupelling: 

Sample No. 1. Check. Sample No. 2. 


Ag+Au.49005 - 49 OI 5 .49032 

Au from parting.01000 .00996 .01002 

Ag.48005 .48019 .48030 

Ag lost in cupelling.oo^oo 

Au “ “ “ .00004 


Percentage Ag lost.— °^°° = 1.03 

.48519 ^ 


Therefore .48005 (Ag found in sample No. 1) must equal 
98.97%. 

Therefore 98.97 : 100 :: .48005 : #=.48505 

Fineness of No. 1 in silver = =072.82 

.49860 

Percentage Au lost (■ OOQO d \ = ^ 

\.1000 / 

'Therefore 99.6 : 100 :: .01000 : #=.01004 

Fineness of No. 1 in gold = - '- 010Q ^ = 20.13 


Impurities = .7% 


.49860 


The fineness of sample No. 2 is calculated in the same way. 

Loss of Silver in Bullion containing Gold.—The following 
results, taken at random, will give an idea of the losses found 
by students when assaying bullion. 

Bullion taken varied from .4500 to .5040 grammes. Lead 
used, 3 to 6 grammes. 



















BULLION. 

20 

Fineness 
in Silver. 

Fineness 
in Gold. 

Total Loss of Silver, i e., 
by Volatilization and 
Absorption by Cupel, 
in Per Cent 

800+ 

50 + 

I *75 

972+ 

22+ 

• 5 i 

923 + 

39 + 

.60 

998 

2 

1.09 

974 + 

20+ 

.78 

969+ 

25 + 

.78 

996 

3-51 

.90 


Wet Methods.—When the Gay-Lussac method with salt or 
the sulphocyanide method are employed, the bullion is often 
assayed by fire to determine its approximate fineness. The fol¬ 
lowing table is taken from Furman’s Practical Assaying. 


If preliminary assay of 
.500 grammes gives 

The silver to be used 
in check is 

C.P. Lead. 

Bullion to be used 
in Second Method. 

. 4900 grammes Ag 

.49500 grammes 

5 grammes 


.4800 

U U 

.48500 

5 


• 475 ° 

cc ic 

.480 

5 

1.042 grammes 

.4500 

cc cc 

.455 to .460 “ 

7 

1.091 *' 

.4250 

cc cc 

• 43 ° to -435 “ 

8 

1.156 “ 

.400 

cc a 

.405 to .410 “ 

10 

T.227 “ 

•375 

n cc 

.380 to .385 “ 

11 

1.307 " 

• 35 ° 

cc cc 

•355 to - 3 6 ° “ 

12 

1-399 

•325 

cc ci 

•330 to -335 " 

13 

1.504 

• 3 °° 

n cc 

.305 to .310 “ 

15 

1.610 

•250 

Cl n 

.255 to . 260 “ 

17 

1.922 “ 

. 200 

cc ci 

. 205 to .210 “ 

19 

2.380 

.150 

«< K 

.155 to .160 “ 

20 

3-125 


Silver Bullion. Volhard’s Wet Method.—This method depends 
upon the total precipitation of the silver in a strongly acid solution 
(HN 0 3 ) by means of a standard solution of potassium sulphocy¬ 
anide (KCvS) or ammonium sulphocyanide (NH 4 CyS). The end 
point is shown by the supernatant clear liquid becoming a per¬ 
manent reddish-brown color (ferric sulphocyanide), when a solu¬ 
tion of ferric alum or ferric sulphate is added to the silver solu¬ 
tion as an indicator. 

The solution of KCyS can be made up in either of two ways. 

First. From the reaction AgN 0 3 +KCyS = AgCyS+ KN 0 3 
we find that 97.17 parts of KCyS will precipitate 107.93 P arts 
of Ag. It follows that 1 gramme of Ag will require .9003 grammes 
of KCyS. Now it has been found convenient to make the KCyS 




















20 8 


NOTES ON ASSAYING. 


solution of such strength that i c.c. will equal io milligrammes 
of Ag. 

If, therefore, we dissolve 9.003 grammes of KCyS in H 2 0 and 
make the solution up to 1000 c.c., each c.c. of solution will, 
according to the reaction, represent 10 milligrammes of Ag. 

The KCyS is, however, very deliquescent, and it is difficult 
to weigh it out accurately; therefore the best way to do is to 
have a No. 1 beaker and weigh into it a little over 9 grammes 
of KCyS, dissolve this in distilled H 2 0 , filter if necessary, and 
make up to 1 litre. 

The indicator may be either a 10% solution of ferric sulphate 
or, better, a saturated solution of ferric ammonium sulphate 
(ferric alum). Make up about 100 c.c. of either; next weigh out 
two portions of C.P. silver of about .5 grammes each. Dissolve 
in 50 c.c. of HN 0 3 (sp. gr. 1.2) and boil to drive off nitrous fumes. 
Dilute with H 2 0 to 200 c.c., allow it to get perfectly cold, and 
add about 5 c.c. of the indicator solution. Titrate and stir con¬ 
stantly. The end-point is a permanent light reddish-brown 
color, and the supernatant liquid should show no cloudiness. 

The KCyS solution will probably be too strong, more than 
9 grammes of the salt having been weighed out. It is an easy 
matter now to calculate the amount of H 2 0 which must be added 
to the KCyS solution in order to make 1 c.c. of this solution 
equal to 10 milligrammes of silver. 

After having added the necessary amount of water, ’ titrate 
the second C.P. silver solution. The KCyS ought now to be of 
such strength that each c.c. is exactly equal to 10 milligrammes 
of silver. If this is not the case, the necessary correction should 
be made from the second titration and either water or a few crys¬ 
tals of KCyS added to the solution. Titrate again. 

Second Method for making up the KCyS. —Weigh out a little 
over 9 grammes of the salt and make up to 1 litre. Next weigh 
out accurately three or more portions of C.P. silver of about 
J gramme each and titrate each of the solutions. They should 
agree very closely, and from them the value in silver of each 
c.c. of KCyS is calculated. 

Bullion Analysis. —If one prefer to take large amounts, 


BULLION. 


209 


weigh out accurately two or more portions of 1 to 2 grammes 
each and titrate them separately; or weigh out 1 gramme or 
over, dissolve in 100 c.c. HN 0 3 (sp. gr. 1.20) or 30 c.c. of strong 
acid (sp. gr. 1.42), boil to drive off nitrous fumes, and make up 
to 500 c.c. in a graduated flask. Shake the flask well, take out 
portions of 200 c.c. each and titrate. The results should agree 
within .05 c.c. 

The best method, however, seems to be to weigh out as closely 
as possible two or more portions of \ gramme each, dissolve in 
10 c.c. of HN 0 3 , boil until nitrous fumes are driven off, cool , and 
add 50 c.c. of water and 5 c.c. of the indicator solution. Titrate 
as usual. The solutions must be cold and entirely jree jrom 
nitrous fumes , and the amount of indicator added must be the 
same in each case. 

The KCyS does not change if kept cool and away from the 
sunlight. If upon titrating, the red color disappears, it is prob¬ 
ably due to the presence of nitrous acid or AgCl. Nitrous fumes 
and HN 0 3 destroy the color in hot solutions. Copper may be 
present up to 70% and not interfere with the titration. Beyond 
this amount, the copper sulphocyanide thrown down obscures the 
end-point. Cuprous sulphocyanide is insoluble in the solution. 
Cupric sulphocyanide is soluble in the solution. Pb, Cd, Bi, 
Zn, Fe, Mn, As, Sb, Sn, and Th do not interfere with the titra¬ 
tion. Hg, Co, Ni, Cl, and palladium interfere with it. 

Results should agree to within .5 at least; that is, if one comes 
988.5, the other should not be lower than 988. 

Recovery of Silver from Solutions.—All the silver nitrate and 
silver sulphate solutions from parting, as well as all the solutions 
and residues from volumetric work, should be saved and the silver 
recovered. 

From Silver Nitrate.—When HC 1 or a soluble chloride is 
added to any soluble salt of silver, except the hyposulphite, a 
precipitate will be thrown down. 

This precipitate is soluble in NH 4 OH, sodium hyposulphite, 
sodium chloride, potassium cyanide, tartaric acid, and soluble 
sulphites. Owing to its solubility in a strong brine, HC 1 seems 
a more suitable precipitant than salt. 


2 IO 


NOTES ON ASSAYING. 


Have the solution boiling before adding the precipitant, and 
after adding it stir well until the AgCl collects in lumps, settles, 
and leaves a clear supernatant liquid. Filter and wash thor¬ 
oughly with hot water by decantation. 

The silver may be recovered from the AgCl in several ways; 
among the best are the following: 

1. Place the AgCl in a vessel, cover with water, add strips 
of zinc and sufficient H 2 S 0 4 or HC 1 to start action on the zinc: 

Zn+H 2 S 0 4 = ZnS 0 4 + 2H; 

2H+ 2 AgCl = 2Ag+ 2HCI. 

When the AgCl has all been changed to metallic silver, indi¬ 
cated by its dark appearance, wash with hot water until free 
from zinc salts, filter, and melt in a graphite crucible. 

2. Fuse the AgCl with sodium carbonate: 

2AgCl+Na 2 C0 3 = 2Ag + 2NaCl+ C 0 2 + O. 

3. Mohr's Method. —Mix the dry AgCl with 1 / 3 its weight of 
resin and fuse in a crucible glazed on the inside or a graphite one. 
The hydrogen reduces the AgCl to Ag. 

Organic matter, cane and grape sugar also decompose the 
AgCl with a reduction of metallic silver. 

4. 4 AgCl + 2CaO + C = 4Ag + 2CaCl 2 +C0 2 . 

From AgCyS.—Filter the solutions, wash and dry the AgCyS. 
Mix with ten times its weight of sodium carbonate, and fuse with 
a little borax in a crucible glazed on the inside. 

It is necessary to use a very large excess of soda, otherwise a 
silver matte (Ag 2 S) will be formed besides the silver button. The 
reaction is probably as follows: 

2AgCyS+ 3Na 2 C0 3 = 2Ag+ 2NaCy+ 2Na 2 S + 3C0 2 + 3O. 

Gold Bullion.*—Experiments at the mints seem to show that 
the amount of gold lost in cupellation increases with the amount 
of lead used, and decreases as the silver in the alloy increases. For 
this reason, as in the assay of silver bullion, we aim to use the 


* Mitchell’s Assaying. 

T. K. Rose, Limits of Accuracy of Bullion Assay. 
Eng. and Mining Journal , Feb. 12, 1898. 




BULLION. 


21 I 


smallest amount of lead that will oxidize and get rid of the impuri¬ 
ties in the bullion, and which, at the same time, will collect the 
•sample or drillings in a good button when the assays first begin 
to drive. Copper also influences the loss and seems to have 
a greater affinity for gold than it has for silver. 

When the bullion contains this metal add 2 \ to 3 times as 
much silver as gold and use the following proportions of lead 
(Mitchell’s Assaying, page 759): 


Gold, 

Parts in 1000. 

Copper, 

Parts in 1000. 

Amount of Lead required when 

1 Gramme of Bullion is used. 

1000 

900 

800 

700 

600 

5 00 

400 to IOO 

0 

IOO 

200 

3 °° 

400 

5 00 

i part 

10 parts - 
16 “ 

22 “ 

24 “ 

26 “ 

7a “ 

1 gramme alloy 

10 grammes lead 






The assay is made practically in the same way as in the silver 
bullion. First we make a preliminary assay to determine the 
approximate fineness. Weigh out, say, .5000 grammes, also 2J to 
3 times as much (C.P.) Ag, and wrap in some (C.P.) Pb. Cupel 
at as low a temperature as possible (feather litharge crystals), 
remembering that towards the end we must have a'slightly higher 
temperature than in the silver bullion, to insure a good “blick.” 
Gold melts at 1064° C. and silver at 961.5° C. Clean the but¬ 
ton carefully and weigh. 

Anneal button, hammer, anneal again, hammer, and finally 
either hammer flat or else roll out into a thin strip. Part in 
the usual manner in HN 0 3 and weigh the resulting gold. 

Suppose our preliminary assay shows: 

.39000 grammes of gold =78% or 780 fine. 

.010 “ of silver = 2% or 20 “ 

.100 “ loss and impurities 

(for instance, copper) = 20% 

From this we can make up our check, which should corre¬ 
spond to the bullion as nearly as possible. That is, if we take 

. 5000 grammes of bullion, 

1.1900 ‘ 4 of C.P. silver (.395 X 3), 










212 


NOTES ON ASSAYING . 


and wrap in 8 to io grammes of C.P. lead-foil (780 fine; .*. the 
Pb=i6X.5, page 211); our check should consist of 

.39500 grammes C.P. gold (This is supposing that J gramme 

of gold loses about .005 during 
cupellation.) 

1.2000 “ C.P. silver (1.19+.010 in the bullion.) 

.0950 “ C.P. copper 

Weigh out two portions of the bullion and wrap each in 8 
grammes of C.P. lead. Drop all three buttons into three hot 
cupels and conduct the cupellation so as to have feather litharge 
crystals. Weigh all three buttons carefully and part the check 
as well as the other buttons. Calculate the results as per pages 
203 and 205. 

Report the results in fineness to second place of decimals as 
in the silver bullion. For instance, 993.63 fine. 

If we wish to determine the silver in the bullion also, especially 
where it is present in very small amount, we shall have to use one 
of the wet methods. See also article by Mr. Cabell Whitehead, 
Chemical Section, Franklin Institute, Sept. 15, 1891, “Use of 
Cadmium for Assaying Gold Bullion.” 

At the mints, after the approximate fineness is obtained, just 
the right amount of silver is added to the gold, both are wrapped 
in C.P. lead and cupelled. The resulting button is rolled out 
thin, coiled into a roll, and placed in a small platinum capsule 
having openings in it. A dozen or more of these capsules are 
held in a platinum tray, which is then immersed in HN 0 3 . 
Three strengths of acid are used in parting—1.12, 1.18, and 
1.27—and each boiling occupies 5 to 10 minutes. The washing 
with water is performed in the same trays, which are finally heated 
in a muffle. Each Pt capsule should contain a little roll of gold. 


CHAPTER VII. 


ASSAY OF ORES FOR COPPER AND TIN. 

COPPER ASSAY. 

Copper melts at 1084° C. Sp. gr. 8.8 to 8.9. Atomic 
weight 63.5. Copper determinations can, of course, be made 
most accurately by some one of the wet methods; still, when the 
ores are of quite a uniform grade, or where the analysis or 
character of the gangue is fairly well known, very satisfactory 
results can be obtained by the fire assay. It is especially adapted 
to places where laboratory conditions are poor and chemicals 
are not at hand. In the Lake Superior region, where the cop¬ 
per exists in the rock in a native condition, the assays of the 
products from the mills, many of which run over 60% copper, 
are made by the fire assay, and the results not only check closely, 
but are nearer the true value of the material than those obtained 
by wet analysis, because a much larger amount of the sample 
(50 to 100 grammes) can be used for a determination. 

Copper ores may be divided as follows: 

1. Sulphide Ores. 

Chalcopyrite, Cu 2 S,Fe 2 S 3 (sp. gr. 4.1 to 4.3). 01 = 34^%, Fe 

= 3 °i> S = 35%. 

Erubescite, or Bornite, purple ore, 3Cu 2 ,S,Fe 2 S 3 (sp. gr. 4.9 to 5.4). 
Cu = 55 i%, S = 28.1%. 

Chalcocite, Cu 2 S (sp. gr. 5.5 to 5.8). 01 = 79.8%. 

Covellite, indigo copper, CuS (sp. gr. 4.59 to 4.64). 01 = 66.4%. 

(Due to the weathering and alteration of chalcocite.) 
Enargite, 3Cu 2 S,As 2 S 5 (sp. gr. 4.43 to 4.51). 01 = 48.3%. 

These ores are of course seldom free from iron pyrites or 
other sulphides, and they may be also associated with arsenical 
and antimonial minerals and compounds. 


♦ 


213 


2 T 4 


NOTES ON ASSAYING. 


2. Oxide and Carbonate Ores. 

Cuprite, red oxide, Cu 2 0 (sp. gr. 5.85 to 6.15). Cu = 88.8%. 
Tenorite, or Melaconite, black oxide, CuO (sp. gr. 5.82 to 6.2)^ 
Cu = 79.8%. 

Malachite, green carbonate, CuC 0 3 ,Cu( 0 H) 2 (sp. gr. 3.9 to 4.03). 
Cu = 57.46%. 

Azurite, blue carbonate, 2CuC0 3 .Cu(0H) 2 (sp. gr. 3.77 to 3.83). 
Cu = 55 - 2 9 %- 

Chrysocolla, silicate, CuSi0 3 -f2H 2 0 (sp. gr. 2 to 2.23). Cu 
= 36.18%. 

3. Native Copper Ores. 

Before making the fusion, the ore to be tested must be in the 
condition of either Class 2 or 3, for if sulphides, sulphates, or 
any other of the compounds in Class 1 are present, a matte will 
be obtained as well as a copper button. The old English method 
(Cornish) of assaying a sulphide ore included the following steps.*: 

1 st. Concentration to a regulus or matte. 

2d. Driving off most of the sulphur by roasting. 

3d. Reduction of black copper by fusion. 

4th. Refining the black copper. 

This necessitates a number of steps, and the following method 
will be found to simplify the process, two steps being used in 
place of four: 

Class I. Sulphide Ores.—These ores if smelted directly would 
give a matte; therefore take from 50 to 200 grammes of the ore 
(through 30- or 40-mesh sieve) and place in either a clay or iron 
roasting-dish. If the latter is used, it must be well coated with 
ruddle (Fe 2 0 3 +H 2 0 ) or chalk before using. Place this in a 
muffle which is hardly red (have the coke to only the bottom of 
the muffle) and heat it very slowly, to avoid caking the ore. Stir 
the ore constantly at first (see notes on Gold Assay, page 132). 
The object of this work, as in the gold work, is to obtain a dead 
roast, i.e., a roast in which neither sulphates nor sulphides are left 
in the ore. Ij lime is present , some oj it will be lejt as sulphate 
(2FeS-f CaC 0 3 + 80 = Fe 2 0 3 + CaS 0 4 + S 0 3 + C 0 2 ); everything 





ASSAY OF ORES FOR COPPER AND TIN. 


215 


else will be in the state of oxides. We shall probably have 
CuO, Fe 2 0 3 , Fe 3 0 4 , Si 0 2 , etc., in the ore at the end of the roast. 

The following are some of the reactions: 

Cu 2 S,FeS 2 +30 = 2C11S +FeO + S 0 2 ; 

2C11S+ 20 = Cu 2 S + S 0 2 ; Cu 2 S + 40 = 2Cu0 + S 0 2 ; 

2Fe0 + 0 = Fe 2 0 3 ; 

Cu 2 S + 2 CuS0 4 = 2 Cu 2 0 + 3S0 2 . 

Some of the sulphates and arseniates formed during the 
roast may be reduced by the addition of carbon, and the sul¬ 
phides and arsenides so formed are then broken up with the 
evolution of S 0 2 and As 2 0 3 . Some sulphates, like those of iron 
and copper, may also be broken up by heat alone, forming CuO 
and S 0 3 , the latter breaking up into S 0 2 and O: 

CuS 0 4 = Cu 0 + S 0 3 ; 

2FeS0 4 = Fe 2 0 3 -f- S 0 2 -f- S 0 3 . 

Sulphate of lime cannot be broken up by either method. 
The final temperature should be almost as high as when scorifying. 

After the dead roasted ore has been cooled and weighed , 
sift it through a sieve to remove any scales or lumps. If either 
of these is present, grind through the same sieve the ore was 
originally put through (30 or 40 mesh) and mix thoroughly with 
the rest of the ore. 

The next step is the same as No. 3 in the English method, i.e., 
to smelt the ore with suitable fluxes and obtain a button of black 
copper. If a matte results in addition to the black copper, the 
ore contains lime or it is not sufficiently roasted. Roast a fresh 
portion of the ore, or else roast the matte and smelt the roasted 
product as if it was an ore. 

Fusion.—Applicable also to ores of Class II. In making the 
fusion, the following should be borne in mind. 

1 st. The slag should be liquid, and as nearly neutral as pos¬ 
sible. 

2d. The amount of flux should be as small as possible, and 
the temperature so high as to have the whole assay finished in 
20 to 30 minutes at the outside. If the assay is kept in the fire 
too long, iron and other impurities are apt to be reduced. 


2 l6 


NOTES ON ASSAYING. 


3d. All reagents, such as soda and argols, should be free from 
sulphur; for this reason use cream of tartar, in place of argols, as 
a reducing agent, and the best of carbonate of soda. 

If these contained sulphur, there would be a copper matte 
formed during the fusion, and the object of the roasting would 
be rendered useless. 

The soda and borax should be melted in an iron ladle or kettle 
before using. This drives off the water present, and the fusion 
for copper is not only made more quickly but more quietly. 

The following charges may be tried. (Use E or F crucibles.) 

A B c 


Ore, 

grammes. .. . 

25 

30 

25 

0 ^ 

Cream of tartar, 

u 

10 

20 

2 charcoal 

2 « 

► C") - 

Soda, 

(( 

25 

10 

3 ° 

cr 3 

1—• 

CD 

Borax glass, 

u 

4 

10 

10 silica 

• 

0 


Use no cover of salt. 

Charge A is for an ore where the gangue is acid. 


Charge B is more of a neutral charge, and C is for an ore 
where the gangue tends to be basic. 

These charges may not answer for every ore, but having made 
two fusions, one of A and one of B, and seen the resulting slag 
and button, it is easy to make changes in the next fusion. That 
is, if the slag is glassy and too acid, add some basic flux like soda 
or iron oxide; if too basic, add silica or borax glass. If the slag 
is red, due to Cu 2 0 , it is either too acid or insufficient reducing 
agent has been used. 

The ore and fluxes may be mixed in the crucible, and the 
crucible then heated, or the crucible may be heated red-hot and 
the charge then added. The latter method makes the shorter 
fusion, but the charge is apt to dust when put in a hot crucible. 

Remove the crucible from the fire, allow to stand and break 
when cold. 

Weigh copper button and calculate the percentage in the raw ore. 

Unless an ore contains lime, duplicate results agree very 
closely, and should come within .2 per cent of each other and 
.3 per cent of the wet analysis. 

Abroad, the copper buttons obtained in a fusion are often 
refined by putting the button in a crucible that has been pre¬ 
viously used, or one that has been glazed with borax glass, cover- 



ASSAY OF ORES FOR COPPER AND TIN. 


217 


ing it up and heating it red. As soon as the copper is melted, 
remove the cover, add 10 grammes of refining flux, replace the 
cover, and in a few minutes remove the crucible from the fire. 
Either allow fusion to cool in the crucible, or else pour. 

Refining Flux. —1 part, by measure, of salt; 2 parts, by meas¬ 
ure, of cream of tartar; 3J parts, by measure, of nitre. Mix 
thoroughly. 

Class II. —These ores, if entirely free from sulphides , require 
no roasting and are fused directly by some one of the charges 
given under Class I. 

Class III. Ore Carrying Native Copper. —Ores belonging to 
this class, for instance those from the Lake Superior region 
may be treated by the following method, which has the great 
advantage over others in that it is well adapted to large amounts 
of rich ore or concentrates in a coarse condition. The amount 
used for assay is rather impracticable for chemical work, and if 
a small amount is taken, it is liable to be an incorrect sample. 
If this small amount happens to represent an accurate sample, 
the ore will necessarily have to be pulverized much finer to obtain 
satisfactory results, and the metallic particles render this very 
difficult. 


The fluxes vary according to the gangue or the foreign matter 
in the mineral. In “ Modern Copper Smelting ” Dr. E. D. Peters 
gives the following: 


No. 

Mineral 

Per 

Cent 

Cu. 

Weight 

in 

Grains. 

Borax 

Glass, 

Grains. 

Soda, 

Grains. 

Sla.g, 

Grains. 

Potas¬ 
sium Bi¬ 
tartrates, 
Grains. 

Sand, 

Grains. 

Iron Ore, 
Grains. 

1 

92 

IOOO 

60 

55 

200 

3 °° 



2 

86 

IOOO 

60 

60 

180 

3 °° 



3 

60 

5 00 

IOO 

80 


3 °° 



4 

33 

5 00 

I 5 ° 

160 


3 °° 

150 


5 

20 

5 00 

190 

200 


3 00 

170 


* 

35 

500 

140 

140 


3 °° 


IOO 

t 

5 to 20 

5 00 

200 

200 


3 °° 




* Calumet and Hecla tail-house mineral. + Rich slag from refining. 


“The percentage of slag-forming materials being so small in 
Nos. 1 and 2, it requires but a slight amount of borax and soda 
to flux them, while an addition of neutral slag is necessary to 
protect the molten copper. A smaller quantity of the ore is 



























NOTES ON ASSAYING. 


2 iS 

weighed out in the succeeding assays, as they are so poor in copper 
that a large amount of flux is required by the great quantity of 
gangue, so that the capacity of the ordinary crucible would be 
greatly exceeded if 1000 grains were used.” 

“No. 3 mineral contains just sufficient ferric oxide to form a. 
good slag with the mixture given; while in Nos. 4 and 5 this sub¬ 
stance, as well as metallic iron, increases to such an extent as to 
require the addition of a considerable proportion of sand to flux 
this base and to prevent the adulteration of the button with metal¬ 
lic iron. The sample of Calumet and Hecla tail-house mineral 
given is typical of the treatment of very silicious material. There 
is nothing remarkable in the considerable proportion of borax 
(an acid flux) used with even highly quartzose ores; for, in addi* 
tion to the fluxing powers of the soda that it contains, a borosili- 
cate is very much more fusible than a simple silicate. No pecu¬ 
liarities exist in the execution of this assay; the ore and fluxes 
are thoroughly mixed on glazed paper and covered with a thin 
layer’of potassium bitartrate, after being poured into the crucible. 

• “The results obtained by this method are surprisingly accurate. 
Duplicate determinations of the lower-grade samples seldom vary 
more than 0.1 or 0.2. A difference of 0.4 per cent is a rare occur¬ 
rence, even in the higher classes of mineral, where the size of the 
metallic fragments renders the sampling and even the weighing 
out of a correct assay a matter of some uncertainty.” 

See also an article by Mr. G. L. Heath in the Engineering and 
Mining Journal , April 20, 1895, “Copper Assaying as Used in the 
Lake Superior Region.” 

Copper ores, ingots, cakes, and bars are generally bought on 
the dry assay, and the dry assay is considered to be what the wet 
analysis gives, less 1.3 per cent. That is to say, if the wet analysis 
of an ore gives 10 per cent copper, it is settled for on the basis of 
8.7 per cent. 

The difference between two wet -analyses should not exceed 
.2 of 1 per cent. 

In actual smelting operations the waste slag often carries less 
than ^ per cent copper, but it is safe to figure on a loss of .75 
per cent. 


ASSAY OF ORES FOR COPPER AND TIN. 


219 


ASSAY OF ORES FOR TIN. 

Tin melts at 232 0 C. Atomic weight = no. Sp. gr. = 7.25. 
Has the property of crackling or creaking when bent. Tin is 
found in few localities compared with the other metals already 
taken up, and is seldom discovered in the metallic state. It 
occurs both in veins and in alluvial deposits. The most im¬ 
portant ore is cassiterite (Sn 0 2 ) or tinstone, and this is the source 
from which most of the tin of commerce is obtained. The chief 
deposits are those of the Straits Settlements, Islands of Banka 
and Billiton, Bolivia, Australia, England, and Saxony. More 
than three-fourths of the world’s production comes from the 
first three, which are alluvial deposits. The others are vein 
deposits. Stannite (sp. gr. 4.3-4.52), a compound of Sn, S, Fe, 
Cu, and sometimes zinc, is also found, especially in South 
America, though it is not as common as cassiterite. 

The color of the oxide may be black, brown, reddish yellow,, 
red, and brownish white. The streak is white to brownish. 
When pure the ore contains 78.67% of Sn. Sp. gr. = 6.8 to 7.1. 
The impurities most frequently associated with the oxide are 
pyrite, arsenopyrite, wolframite (tungstate of Fe and Mn), chal- 
copyrite, titaniferous iron, columbite, iron oxide, tourmaline, and 
sometimes blende and galena. To determine whether a mineral 
is Sn 0 2 , fuse some of the fine mineral in a porcelain crucible or 
similar vessel with 3 or 4 times the amount of KCN and dissolve 
mass in water. A tin globule or globules will be found if the 
mineral is Sn 0 2 . Or fuse a mixture of the mineral, sodium car¬ 
bonate, and charcoal on charcoal in the reducing flame of a lamp- 
or candle. 

When in veins the gangue generally consists of granite, slate, 
syenite, quartz, or feldspar, and often carries garnets and zircons. 
Fluorspar is also frequently present, and by some is considered 
a good indicator of tinstone.. 


220 


NOTES ON ASSAYING. 


Portions of the deposits are often very rich, but the average 
of the ore, whether from veins or placers, carries only i% to 5% 
Sn 0 2 . On this account, samples can very rarely be assayed by 
fire or even analyzed in the wet way directly. 

Owing to its high specific gravity, however, we can resort to 
washing and concentration, thus separating it from the gangue 
and some of the other impurities. Wolframite, unfortunately, 
has a specific gravity (7.2 to 7.5) slightly higher than tin oxide, 
which necessitates a special purification later on, when this min¬ 
eral is present. 

The following table from Mitchell’s Assaying will show why 


it is necessary to concentrate 
.much as possible. 

or free the 

ore 

from its 

gangue as 

''Ore used in grammes. 


10 

10 

10 

10 

: SiOo present in grammes. 

. 2 -5 

6.6 

10 

!5 

30 

Tin obtained by fire assay. 

. 52% 

43 % 

28% 

10% 

0% 


Tin oxide has a great affinity for silica, for it has the property 
of acting both as an acid and as a base, and in this case acts as a 
base. If the tinstone carries much iron oxide, this has to be 
removed with acids, otherwise the resulting tin will not collect 
in a button, but will contain iron and be a porous and magnetic 
mass. 

An ore carrying 4 per cent Sn is considered a fine ore; so it 
can be readilv seen that one must first resort to concentration 

s' 

before making a fire assay. This is also the better plan even 
before attempting a wet analysis, unless the sample submitted 
for analysis is very rich. The steps in the assay are as follows: 

1st. Concentration. 

2d. Roasting the concentrates. 

3d. Panning the concentrates and boiling in aqua regia. 

4th. Panning the concentrates again. 

5th. Assaying the final concentrates. 

If the concentrates obtained from the first panning are very 
pure, some of the later steps may be omitted. 

Concentration.—Take 500 to tooo grammes of ore, crushed 
through at least a 40-mesh sieve. If it is crushed too fine, the 
Sn 0 2 will slime badly; still it must be fine enough to liberate 





ASSAY OF ORES FOR COPPER AND TIN. 


221 


the Sn 0 2 from the gangue. Carefully pan or van the ore again 
and again until no more concentrates can be obtained. Do not 
pan down too close , for if a little gangue is left with the con¬ 
centrates it does not matter. The waste matter or tailings are 
thrown away. 

Roasting.—The concentrates consisting of Sn 0 2 , pyrite, and 
whatever heavy material there happened to be in the ore, together 
with a small amount of gangue, are dried and then placed in a. 
clay or an iron roasting-dish. This is next placed in a muffle, 
the bottom of which is hardly red, and slowly heated. When 
the odor of S 0 2 can no longer be detected, the dish is taken out, 
cooled, and a little fine charcoal stirred into the ore. This reduces 
the sulphates, arseniates, and antimoniates to lower forms and 
enables the S, Sb, and As to be set free, and is especially necessary 
when arsenic is present. Roast again and repeat until a dead 
roast is obtained. Everything in the concentrates should now 
be in the state of oxides. They can now be panned to remove 
the oxides of iron and silica and then treated with acid, or they 
can be treated with acid directly and then panned. 

Treatment with Acid.—Sn 0 2 is insoluble in aqua regia; 
therefore by boiling the concentrates in this we practically get 
rid of everything except some Si 0 2 , Ti 0 2 , and compounds insoluble 
in aqua regia. If much Si 0 2 is present, pan again. In some 
cases it is well to grind the tailings fine from this concentration 
and pan again. Dry the total concentrates, weigh and grind 
through an 8o-mesh sieve. 

Assaying.—The concentrates are now ready for assaying, 
and this may be done by various methods. Among the most 
approved are the following: 

First Method. —(Levol’s Cyanide of Potash.)—This method has- 
always given me the most satisfactory results, so it is placed first.. 
On clean ores or concentrates it is very accurate, but when the ore 
or the concentrates contain much foreign matter the assay is. 
rendered much more difficult and the time of fusion has to be. 
increased. 

Take 5 or 10 grammes of concentrates and mix with four 
times as much KCy, C.P. ( KCy is a deadly poison I) 


222 


NOTES ON ASSAYING. 


Have a good layer of KCy in the bottom of the crucible, 
next put in the mixture of concentrates and KCy, and then place 
a layer of KCy on top of all. 

Crucible is a Battersea A or similar close-grained crucible. 

2KCy + Sn 0 2 = Sn + 2KCyO. 

Layer of KCy (through 8). 

Concentrates and KCy mixed. 

Layer of KCy (through 8). 

Heat very slowly at first and just juse to reduce the Sn 0 2 to 
Sn and then keep just jused for 20 to 30 minutes. Increase tem¬ 
perature 10 to 15 minutes longer and then take from the fire, 
tap gently, and transfer to some place where the fumes will not be 
carried into the laboratory. The purer the Sn 0 2 is the shorter 
the period of fusion; with some ores the fusion has been com¬ 
pleted in 10 minutes with good results, but as a general thing 
the ordinary concentrates will require the above time to make the 
fusion satisfactory. Certainly the more impure the concentrates 
are the longer the fusion must be. Never have the fusion boil , 
jor low results will be obtained. 

Allow the crucible to become perfectly cold. (Tin melts at 
232 0 C.) Break the crucible and place it and contents in a dish 
and cover with water. If the decomposition of the ore is com¬ 
plete, we shall have a nice bright tin button, with perhaps a few 
prills or small buttons. If the fusion is incomplete, we shall find 
some ore still undecomposed. The great advantage of this 
method of assay is that although one works with an expensive 
and poisonous reagent, one can see at the end of the fusion 
what has actually been done. 

The KCy should be C.P. The impurities are generally 
chloride, carbonate, and cyanate of potassium or of sodium. 

Some assayers recommend a cover of salt, but I consider 
.a layer of KCy amply sufficient. 

Ten grammes of concentrates have always given me higher 
results than 5. Report any and all results in percentage on the 
original ore used. Results on concentrates and original ore 







ASSAY OF ORES FOR COPPER AND TIN. 


223 


should agree very closely and ores carrying one-half per cent Sn 
can be assayed satisfactorily by the foregoing method of concen¬ 
tration. 


Second Method (German Assay). 

Rich ore or concentrates (through 80) 5 5 10 ) 

Charcoal (through 80). i i 2 f Mix in bottom of crucible 

Flour . 5 3 5 ) 

Bicarb, soda. 10 5 — > Mix and put on top 

Bicarb, potash. 10 5 20 ) 

Borax glass. — ^ inch 2 grammes 

Cover of salt in each case, and a small piece of charcoal on top. 


Heat very slowly at first, say about 20 minutes, and then fuse 
until charge is quiet and foaming ceases, about 1 to 1^- hours. 
The results are good, but the button, owing to the high heat, is 
apt to contain a small amount of iron. If a small chip cut off 
the button is not magnetic, the amount of iron is very small. 

Third Method .—This method is given by Mitchell, who uses 
larger amounts of material than I give, but in the same ratio. 
Personally I have never had any success with it. 


Ore or concentrates 

Argols. 

Sodium carbonate.. 
Rime. 


12 grammes 

U 


3 

9 


«« 

it 


Jh 

* £ 
v .2 o 

* a 2 

o 
G 


Reactions. 

Na 2 Co 3 -f- heat = Na 2 0 + Co 2 ; 
Na a O+C = 2 Na-f CO; 
4Na-f-Sn0 2 =2Na 2 0-fSn. 


“Mix well together, place in a crucible which the mixture half 
fills, cover with a small quantity of sodium carbonate and 5 
grammes of borax. Place the whole in the furnace with the 
necessary precautions, raise the heat very gently, and keep it at 
or below a dull red heat for at least 20 minutes, then gradually 
increase until the whole flows freely. Remove the crucible, tap 
it as for copper assay, and allow to cool. When cold, break it, 
and a button of pure metallic tin will be found at the bottom, and 
a flux perfectly free from globules and containing no tin.” 

Fourth Method .—Take 25 to 50 grammes of fine concentrates, 
place them in a graphite or charcoal-lined crucible, and cement 
the cover on firmly, leaving a small opening. Heat at a dull red 
for 15 minutes and then at a bright red for 10 minutes. Remove 
the crucible with care and do not tap. The results are generally 
low, owing to prills and small buttons. 












CHAPTER VIII. 


PLATINUM AND THE PLATINUM GROUP. 

The platinum group of metals consists of platinum, iridium, 
osmium, palladium, ruthenium, and rhodium. 

Iridosmium is an alloy of iridium and osmium. If we divide 
the group by atomic weights, it is seen that platinum, iridium, and 
osmium are very closely associated; likewise palladium, ruthe¬ 
nium, and rhodium. 

The group may perhaps be further divided into platinum and 
palladium, rhodium and iridium, osmium and ruthenium. In 
regard to the metals’ affinity for oxygen, we find that the oxides 
of osmium and ruthenium are volatile, or partly so; that iridium, 
palladium, and rhodium oxidize when heated with free access of 
air, but the oxides break up upon further and higher heating into 
the metal and oxygen. 

Platinum does not oxidize, although it can be thrown down 
from solution as an oxide. Russia and the United States of 
Colombia are the chief sources of supply, but the metal is much 
more widely distributed than is generally known. Although 
occurring most commonly in grains and nuggets in alluvial sands 
and placers, it has been found in rock in place. Fine grains 
have been found in quartz, but the basic rocks, like olivine, ser¬ 
pentine, peridotite, and chromite, are the ones from which most 
platinum has been derived, for pieces of these rocks are often 
found attached to nuggets. 

Platinum grains are grayish white in color, and their specific 

gravity depends upon the other metals or those of the platinum 

224 


PLATINUM AND THE PLATINUM GROUP. 


225 


group associated or alloyed with them. The grains or nuggets 
are seldom pure, and not only contain the members of the group, 
but analyses have shown gold, manganese, iron, and copper. 
The last two are very common impurities and some nuggets have 
carried as high as 17% iron. It is also interesting to note that 
considerable platinum has been found, in several instances, in 
copper sulphides and tetrahedrite. 

Platinum does not amalgamate even when both it and the 
mercury are heated. When immersed in sodium amalgam, the 
mercury seems to adhere to its surface, but it does not really 
amalgamate. This property can often be taken advantage of to 
remove the platinum grains in an ore after the removal of the 
gold by mercury alone. 

The sp. gr. of the grains has been found to be anywhere 
from 13 to 19 and the purity from 50 to 86% Pt. The sp. gr. 
of melted platinum is between 19 and 20, while that of the metal 
when hammered is between 20 and 21.5. 

Sperrylite, an arsenide of platinum (PtAs 2 ), sp. gr. 10.6, is 
the only platinum mineral at present known, and was discovered 
in 1888 by F. L. Sperry in the Sudbury district, Ontario, Canada. 
Since then it has been found in several places in the United States. 
The only other known mineral of the group is laurite, a sul¬ 
phide of ruthenium and osmium. 

The table on pages 226 and 227 gives some of the properties 
of the group, to which have been added silver and gold. 

Qualitative Tests.—If the ore is very low grade, it had bet¬ 
ter be concentrated and the concentrates taken for the test. 
If free gold is present, remove it with mercury. 

Treat the ore or concentrates with aqua regia, evaporate 
several times to a syrup with HC 1 , in order to remove all the 
HN 0 3 . Finally, take up in water and a few drops of HC 1 , boil 
and filter. The Pt should be in solution as bichloride (PtCl 4 ). 
This solution can now be tested for Pt in either of two 
ways: 

1. Take a small amount of the cold solution and add to it 
a strong solution of potassium iodide. If Pt is present the solu¬ 
tion will become a deep red color (platinum iodide). The test 


226 


NOTES ON ASSAYING. 


TABLE OF SOLUBILITY OF SILVER, 



Silver. 

Gold. 

Platinum. 

Iridium. 

Atomic wght 

107-9 

197.2 

194.9 

193 

Specific gr. 

io.4-10.7 | 

is-19. 3 native 
19.3 pure 

Melted 19-20 
Hamm’d 20-21.5 

| 22.42 

Fusibility 

961^° 

1064° C. 

1760° to 1765° 

About 1950° 

Hardness 




Below 7 

Color of 

Brilliant 

Yellow 

Grayish white 

Steel-white 

metal 

white 



Powder 



Black 

Steel-gray 

Heat (air) 

Not oxidized 

Not oxidized 

Not oxidized 

Oxidizes when 




heated in air, 
but oxides break 
up, when fur¬ 
ther heated,into 
metal and oxy- 






gen 

h 2 so 4 

Soluble 

Insoluble 

Insoluble 

Insoluble when 





alloyed with 
silver, if due 
precautions are 
taken 

h 2 so 4 



Partly solu b 1 e. 

Partly soluble 

(fuming) 



Ir lessens the 

when alloyed 




solubility 

with Ag 

HNO, 

Soluble 

Insoluble 

Soluble -when al- 

Insoluble. Partly 




loyed with 

soluble when 




Ag 12, Pt 1; or 

alloyed with 




Agi2,Pt 1,Au 2 

silver 

Aqua regia 

AgCl precipi- 

Soluble 

Soluble. Dilute; 

Soluble when al- 


tates 


1 to 3 is the 

loyed with Pt. 




best. Said to 

Soluble in 




be insoluble 

strong aqua re- 




when alloyed 

gia. Insoluble 




with certain 

in dilute. Com- 




percentages of 

pact iridium is 




iridium or with 

said to be in- 




rhodium 

soluble in all 





acids 

Sodium hy- 





pochlorite 





Fused with 




Oxidizes but does 

potassium 




not dissolve it 

disulphate 




> • > 


















PLATINUM AND THE PLATINUM GROUP , 


227 


GOLD, AND THE RARE METALS. 


Osmium. 

Palladium. 

Ruthenium. 

Rhodium. 

Iridosmium 

or 

Osmiridium. 

191 

106.5 

101.7 

103 


22.48 

11.4 

11-12 

12. I 

18-21.5 

Melted only in 
the electric arc 

About wrought 
iron 

Over 2500° C. 
Higher than 
rhodium 

Higher than Pt 


Above 7 

4 - 5-5 

Below 7 


6.7 

Bluish 

Steel-gray or 
white. Darker 

Grayish white 

Whitish gray 

Tin-white to 
steel-gray 

Bluish 

[than Pt 

Purple-black 


Black 

Oxidizes. 0 s 0 4 

Oxidizes on heat- 

Oxidizes. RuO 

Oxidizes when 


very volatile 

ing, but ox¬ 
ides break up 
into metal and 
oxygen. Metal 
resumes its 
color 

and Ru 0 2 vol¬ 
atile. Ru 2 0 3 
(bluish black) 
not volatile; 
decompo s e d 
by heat 

heated, but 
oxides break 
up into metal 
and oxygen 


Insoluble. In- 

Slightly soluble, 

Insoluble when 

Insoluble. There 

Insoluble 

tensely ignited 

especially 

alloyed with 

are two modi- 


Os is insolu- 

when alloyed 

silver if due 

fications 0 f 


ble in all acids. 
Silver does not 
affect solubil¬ 
ity 

with Ag 

Soluble with dif¬ 
ficulty. Solu¬ 
ble when al¬ 
loyed with Au 
and Ag 

precautions 
are taken 

Rh. Compact 
and precipi¬ 
tated 


Soluble with for¬ 
mation 0 f 

0s0 4 

Partly soluble 
when alloyed 

with A g . 

hno 3 +hno 2 

with ease 

Insoluble 

Compact; insol¬ 
uble. Ppt’d, 
slightly solu¬ 
ble 

Insoluble 

Insoluble 

Soluble. PdCl 2 
and PdCl 4 
separate out 
upon long 
standing 

Soluble 

Slightly soluble. 
Oxides insol¬ 
uble 

Soluble when 
finely divided 
Attacked 

Is said to be sol¬ 
uble when al¬ 
loyed with 

other rare 
metals and 
Cu, but not to 
be when al¬ 
loyed with sil¬ 
ver or gold 

Soluble 

Insoluble 















228 


NOTES ON ASSAYING. 


is very delicate, and if much Pt is present, only a very small 
amount of the solution to be tested should be taken. Heat will 
cause the color to disappear. 

2. To the very concentrated solution add a strong solution 
of NH 4 C 1 or the salt itself, and then a good deal of alcohol. Allow 
to stand for 24 hours at about 8o° C., when, if Pt is present, a yellow 
precipitate of (NH 3 ) 2 PtCl 6 wiL be thrown down. Any gold pres¬ 
ent as AUCI3 will remain in solution. If the ore sperrylite, the 
arsenide of platinum, is to be tested for, take a small portion 
and drop it on a hot piece of platinum-foil: As 2 0 3 will be given 
off, leaving spongy excrescences of platinum similar to the foil. 

Sperrylite is only partly soluble in aqua regia, and is not 
attacked by hydrofluoric acid. 

Quantitative Analysis.*—Quantitative work upon platinum 
ores, especially where other rare metals are present, is very diffi- 
cult, and most of the methods are long and complicated. 


ASSAY OF THE SANDS AND ORES. 

I prefer to divide platinum ores into three classes: 

1. Ores carrying no metallic grains, which can be assayed 
directly. 

2. Ores with value in concentrates but carrying no metallic 
grains, which are too poor to assay without a previous concentration. 

3. Ores carrying metallic grains. 

Class I. Crush these ores through a 120-mesh sieve, assay 
them exactly as if assaying ores for gold, and obtain a lead button 
weighing from 25 to 30 grammes. If ore is very poor take 5 A.T. 
or more and assay as in the case of low-grade gold ores, p. 130. 

Class II. Take 300 or more grammes of ore (through a 30- or 
a 40-mesh sieve), concentrate it, and then assay the concentrates 

* Bibliography of the Pt Group, Dr. J. Lewis Howe, and among others the fol¬ 
lowing: St. Claire, Deville, and Debray, Annals, de Chim. et de Phys. (3), vol. 56 
(1859), p. 385; Dr. H. Pirungruber, Eng. and Min. Journ., vol. 44, p. 256; E. 
Wiatt, Eng. and Min. Journ., vol. 44, p. 273; T. Wilm, Journ. Chem. Soc., vol. 
50 (1886), p. 181; also Journ. Soc. Chem. Ind., vol. 4 (1885), p. 759- Mitchell’s 
“Assaying,” p. 781; Crookes’ “Select Methods,” pp. 446 to 476; E. Leidi£, 
Bulletin Societe Chimique (3), vol. 25, p. 9; E. Leidie and Quennessen, Bulletin 
Societe Chimique (3), vol. 27, p. 179. 



PLATINUM AND THE PLATINUM GROUP. 


229 


as in the assay of concentrates carrying gold. The tailings are 
thrown away. The lead button from this fusion should weigh 
25 to 30 grammes and its treatment is described later on. Figure 
the results on the original ore taken. 

Class III. Ores of this class are the ones most frequently 
met with, and their preliminary treatment must be carefully 
conducted. The aim is to remove the platinum grains as far 
as possible and to make all the grains in the sample into rich 
lead bullion. 

Samples weighing less than 1000 grammes should be crushed 
through a 30- or 40-mesh sieve. Save any pellets on sieve. Amal¬ 
gamate all the ore to remove the gold, and concentrate to a small 
amount of heads. Remove any Pt grains. The heads and tails 


Ore, 900 grammes 
Crushed through 30-mesh sieve 



Melt with 40 grammes 

or more of lead and chill quickly. 

I 

Weigh bullion; take drillings 
and cupel several portions. 

Figure the total Pt in the ore (T) and the lead bullion, and base the final result 
on the 900-gramme sample. 



















23 ° 


NOTES ON ASSAYING. 


should all be ground through an 8o-mesh sieve and pellets saved* 
Assay, as usual, the material through an 8o-mesh sieve. 

The pellets are all put together and melted with lead. This 
bullion should be chilled instantly, to prevent segregation of the 
metals,* drilled or cut up into pieces, and assayed as described 
later on. 

A tree of the treatment is given on the preceding page. 

This method eliminates the question of small samples disi- 
greeing, owing to uneven distribution of pellets. 

Large samples should be treated as follows: Crush 3 to 5 kilos 
through a 30-mesh sieve. Save any pellets. Amalgamate all the 
ore to remove gold. Pan out the amalgam and remove the con¬ 
centrates. Recover all the platinum grains possible from the 
concentrates. The heads and tails are dried, thoroughly mixed, 
and a sample of at least 700 grammes taken. The Pt grains 
so far obtained are weighed, melted together with lead, and results 
based on 3 to 5 kilos. The 700 grammes are treated as previously 
described, and all grains are kept separate, wrapped in lead, and 
cupelled. 

This button need not be analyzed, because the analysis of 
the main lot of pellets for the other rare metals will be sufficiently 
accurate for this or any other residue. 

The work so far has given us, from Classes I and II, lead 
buttons carrying silver and gold, if present in the sample, together 
with platinum and any of the rare metals, or, from Class III, 
rich lead bullion carrying Pt and the rare metals. The advan¬ 
tage of a large button rich in Pt over a small low-grade lead 
button is that drillings can be taken from the former and sev¬ 
eral assays made instead of relying upon one. We can now 
proceed as follows: 

Cupel the lead button (Classes I and II) or the drillings from 
the bullion as usual, except towards the end, when, if much Pt is 
present, the heat must be very high in order to remove the last 
traces of lead. The presence of a small amount of Pt makes the 


*Ed. Mathey, Client. News, vol. 61, p. ill, “Liquation of Au and Pt 
Alloys.’' 





PLATINUM AND THE PLATINUM GROUP. 


231 


final bead crystalline on the surface and often covered with 
irregularities. If very much Pt is present, the button will stick 
to the cupel, be spread out, irregular, and have a gray mossy 
appearance. Such buttons are liable to contain lead, for it is 
extremely difficult in such cases to remove the last traces of 
this metal. 

Care should be exercised in hammering the beads contain¬ 
ing any of the platinum group, for a small amount of lead seems 

to render them brittle at times. 

« 

A. Bauer found that an alloy of 3 parts Pb and 1 part Pt 
was as brittle as glass. 

From now on the difficulties that we encounter are increased 
by the influence which silver and gold , if present , have upon the 
solubility of Pt and the rare metals , as well as the influence the 
rare metals have upon the solubility of each other. 

The best method of procedure seems to be as follows, and I 
am indebted to the following students, who have taken theses 
on the analysis of Pt ores and the rare metals, for much of the 
data: H. B. Litchman and D. C. Picard, class of 1903, and R. 
B. Williams and J. R. Marston, class of 1904. 

Determination of Ag, Au, Pt, Ir, and Iridosmium.—Clean the 
bead, hammer if possible (some buttons containing Pt, silver 
and the rare metals, or a certain ratio of Pt and Ag are brittle) 
and weigh (a). Part in strong H 2 S 0 4 , boiling three times; the 
silver will dissolve, leaving the other metals, if three or more 
parts of silver are present. Much care must be used in this 
parting, for when H 2 S 0 4 is used the Au and Pt are liable to be 
left in a finely divided condition. Furthermore, if iridium is present 
and it is finely divided, it will float on the top of the solution. 
In such cases the H 2 S 0 4 and Ag 2 S 0 4 , after cooling somewhat, 
should be decanted into a casserole containing H 2 0 . Cautiously 
pour hot H 2 0 into the parting-flask, allow to stand some time, 
and again decant, repeat twice more, when the Ag 2 S 0 4 should 
be removed. Filter the contents of the casserole through a 
washed filter. Fill the parting-flask with hot water and trans¬ 
fer contents to an annealing cup or porcelain crucible, allow- 


232 : 


NOTES ON ASSAYING 


ing flask to stand inverted in cup at least five minutes, and occa¬ 
sionally tapping its sides. Wash the contents of the filter and 
transfer it and its contents to the annealing cup with the rest of 
the metals. Ignite the filter-paper cautiously and finally heat 
in the muffle. Cool and weigh. Sometimes the residue sticks 
slightly to the cup, but, as a rule, not badly. This residue is 
b, and the difference between a and b is the silver in the ore. 

If the amount of silver is extremely small, this result can 
serve as an approximate determination, for considerable Ag is 
lost at the high temperature required in cupelling. After the 
Pt and other elements are determined, weigh out a known quantity 
of Ag, add it to the lead button from another assay of the ore, 
run a check at the same time, as in bullion, cupel, dissolve but¬ 
tons in HN 0 3 , and titrate with a solution of salt. Make allow¬ 
ance for the silver added and figure the results as in the assay 
of silver bullion. 

Add to the residue b 12 to 15 times as much silver alone 
or 12 to 15 times as much silver and 1 or 2 times as much 
gold. If gold other than free gold is present in the sample, 
do not add gold; wrap in 6 to 8 grammes of lead and cupel. 
Clean the button, roll out, and drop into some warm HN 0 3 
(1.20 or 1.28 sp. gr.). All of the silver and part of the Pt (75% 
t0 95 %, if gold is present) will go into solution. If gold is used, 
repeat this process twice more, but of course add no more gold. 
If silver alone is used, repeat until all the Pt is dissolved. The 
final residue should consist of gold, iridium, and iridosmium. 
Weigh (c). (c) minus any gold added, subtracted from ( b ), is 

the Pt in the sample. 

The residue c is treated for a few minutes with dilute aqua regia, 


1 part aqua regia 


/3 parts HC 1 \ 
\i part HN 0 3 / 


and 5 parts H 2 0 , 


which dissolves the gold and any Pt that might be present. 
Wash, ignite, and weigh residue d. Residue c, minus any gold 
added, less d is the gold in the sample. Treat residue d with 


PLATINUM AND THE PLATINUM GROUP. 


233 


strong aqua regia, which dissolves the iridium, leaving the iridos- 
mium and rhodium, also osmium and ruthenium, if these last 
have not been volatilized during cupellation. 


EXAMPLES. 


Residue after parting in 

Residue after 

Pt and Au 

After strong 

Ir 

h 2 so 4 

dilute aqua regia 

grammes 

aqua regia 

grammes 

1. .00388 grammes 

Au, Pt, Ir, Rh, and 
Iridosmium 

.00337 

Ir, Rh, Iridos¬ 
mium 

.00051 

.00327 

Rh and Iri¬ 
dosmium 

.00010 

2. .00230 grammes 

.00040 

.00190 

.00035 

.00005 


If gold is absent from the ore and Pt alone is to be deter¬ 
mined, the procedure is as follows: Cupel the lead button as 
described, adding 2} to 3 times as much silver as the amount 
of Pt supposed to be present in the sample. Part the resulting 
bead in strong H 2 S 0 4 and weigh the residue (x), which will prob¬ 
ably be grayish black. Treat with dilute aqua regia, i.e., 1 to 
5, in the proportions just given, or else 1 to 3; the Pt will 
go into solution, and if any residue is left it is probably Ir, 
Rh, Ru, or iridosmium. Dry, ignite, and weigh. The dif¬ 
ference between this and the first weigh (x) is the Pt in the 
sample. 

Care in Coupelling.—Where a large number of samples con¬ 
taining gold and platinum are to be assayed for Pt it is well to 
classify them if possible, according to Pt contents, into rich, 
medium, and poor ores. This will enable one at the time of 
cupellation to place those rich in Pt in the back of the muffle, 
where a high heat is necessary, and those poor in Pt in front, 
w r here litharge crystals can be obtained as usual. If this is 
not done and the rich and poor buttons are scattered about indis¬ 
criminately, the high heat necessary for the rich ones will cause 
such a loss of gold in the poor ones that the Pt results will be 
far too high. 

Where the gold is to be determined, and in all espe¬ 
cially nice work, checks should be run as in the assay of 
bullion. 








234 


NOTES ON ASSAYING. 


The following work, done by Mr. H. B. Litchman, class of i903 > 
is of interest in this connection. Pure lead, silver, gold, and plati¬ 
num were used in the tests. 

Nos. 15, 16, and 17 were cupelled in the middle of a muffle 
heated by gas and then pushed back; the others were cupelled in 
the front and gradually pushed back until they stood at the back 
of the muffle. All of them were left in the furnace 1 minute 
after blicking at a very high temperature. 

Nos. 15, 19, and 22 were treated with 10 c.c. HN0 3 (i. 28 sp.gr.), 
boiled, 10 c.c. more added and the boiling continued. No. 22 
was treated with a third portion. No. 15 did not break up; 19 
and 22 broke up, rendering the solution brown and turbid, due 
to the finely divided Pt. 

Nos. 16, 18, 20, and 23 were treated with 20 c.c. HN 0 3 (1.20 
sp. gr.), boiled, and then 10 c.c. more added. They all gave tur¬ 
bid solutions of a brownish color. 

Nos. 17, 21, and 24 were treated with 20 c.c. HN 0 3 (1.16 
sp. gr.), boiled, and 5 c.c. more added. 21 and 24 broke up, but 
17 did not. All the solutions had to be filtered, the residues thor¬ 
oughly washed, and the filters and contents transferred to porce¬ 
lain crucibles, ignited, and weighed. 

It is very evident from the results in Table I that HN 0 3 will 
not dissolve in one treatment, as it is frequently claimed it will,, 
all the Pt from an alloy of Ag and Pt. 

The results in Table II show the effects of adding gold to 
an alloy of silver and platinum and treating the button with 
HNO s (1.28 sp. gr.). 

All the tests were cupelled near the front of a gas-muffle until 
near the blicking-point, when they were pushed to the back and 
kept there 3 minutes after the colors had disappeared. 

The buttons all indicated the presence of Pt. 

No. 25 was treated with 25 c.c. HN 0 3 (1.28 sp. gr.) and kept 
warm while action lasted, and then boiled. The other buttons 
were treated with 20 c.c. hot HN 0 3 (1.28 sp. gr.), boiled, this solu¬ 
tion decanted and 10 c.c. fresh acid added. After boiling in this 
the residues were thoroughly washed and transferred to porce- 



TABLE I.—SILVER AND PLATINUM ALLOYS. 

The Use of Different Ratios of Pt to Ag and their Solubility in Acids of Different Strengths. 


PLATINUM AND THE PLATINUM GROUP 


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236 


NOTES ON ASSAYING, 


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PLATINUM AND THE PLATINUM GROUP . 


237 


lain crucibles. All the solutions were clear and there was no more 
difficulty in collecting the residues than in the usual parting for 
gold. 

The Pt dissolved was considered the difference between the 
residue left after parting and the Au + Pt originally taken. This 
is not strictly correct, because there is a loss of gold in cupelling, 
but the amount of Pt found in the cupels was practically nothing. 

It will be seen from this series of tests that gold has a remark¬ 
able influence upon the solubility of Pt in HN 0 3 when alloyed 
with silver, and its presence also renders the solutions clear and 
residues readily handled. 

From Table II the best ratio seems to be Pt 1, Au 1 or 2, 

Ag 15- 

The following tables seem to show that, even with this ratio 
of gold and silver to platinum, three treatments with HNO.,. 
(1.28 sp. gr.) are necessary to dissolve all the Pt. 

One more cupellation with Ag and parting in acid will dis¬ 
solve all the Pt and leave a golden-yellow residue. All the buttons 
were cupelled at a high temperature and left in the furnace one 
minute after the colors had disappeared. The parting was done 
by dropping them in 25 c.c. of warm HN 0 3 , boiling the solution 
after action had ceased, diluting to 35 c.c., washing and filtering. 
The filtering was simply an extra precaution, for the solutions were 
clear and colorless. On the other hand, if silver alone is used, 
a great many partings are necessary, the solutions are turbid, 
the residues are extremely finely divided, and filtering is gener¬ 
ally necessary. 

The presence of gold increases the solubility of Pt in HN 0 3 , 
and the per cent of Pt dissolved, when the ratio of 1 or 2 of gold 
is used, is always high, yet it varies considerably, which seems to 
indicate that there is some factor in regard to its solubility yet 
undetermined.* The solutions from the filtrations were freed 
from silver and tested for gold, but none was found. 

The loss of Pt during cupellation is very small, but there must 

* Notes on the Separation of Au, Ag, and Pt, Journ. Soc. Chem. Ind., vol. 22, 
p. 1324, by H. Carmichael. 



23 % 


NOTES ON ASSAYING. 


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PLATINUM AND THE PLATINUM GROUP. 239 

be a loss of gold, and this loss, together with any loss in parting, 
will make the final Pt results too high. 

Too high results for Pt and other rare metals may also be 
.obtained, due to the solubility of these metals in acids when al¬ 
loyed with each other, though not otherwise soluble. 

Experiments in this laboratory by Mr. J. R. Marston seem to 
show the following: 

Alloys of Ag and any of the Pt group must be rolled or ham¬ 
mered thin before treatment with any acid. When H 2 S 0 4 is used 
it must be strong and the button must be boiled in it for some 
time, otherwise some silver will remain undissolved. 

To treat the residue with HNO a , after the H 2 S 0 4 , is not always 
safe, as Pt may dissolve. 

Platinum.—-According to my experiments when an alloy of 
Pt and Pb is treated even with dilute HN 0 3 (1.1 or 1.2 sp. gr.) 
an appreciable amount of Pt goes into solution. 

The residues after treatment are liable to adhere to both 
glazed and unglazed cups. 

According to Winkler,* when an alloy of Pb and Pt is treated 
with HN 0 3 , 13.11% of the Pt is soluble in acid of 1.19 sp. gr.; 
I 3- 2 3% i n ac id of 1.298 sp. gr.; and 14.57% acid of 1.398 
sp. gr. 

The alloy formed seems to be Pb 2 Pt when the lead is in ex¬ 
cess, and PbPt when not. 

Gold and silver alloy perfectly with Pt; other metals of the 
group, with the exception of palladium, are said not to form per¬ 
fect alloys. 

Iridium.—When a silver button, after cupellation, contains 
iridium, the surface has an irregular appearance, similar to that 
given by Pt, but the roughness seems of a finer texture. 

When present in an alloy of Ir+Pt 1 part, Ag 12 to 15 parts, 
the solubility of the Pt in HN 0 3 is diminished. 

It seems to be insoluble in dilute aqua regia, and only slightly 
soluble in aqua regia made up of 1 HN 0 3 and 5 HC 1 or 1 HN 0 3 
and 3 HC 1 . The presence of platinum alone seems to increase 


* Solubility of the Pt in Pt alloys in HNO s , Journ. Chem. Soc. } vol. 13, p. 428. 



240 


NOTES ON ASSAYING. 


the solubility of the iridium. It is soluble in aqua regia of i part 
HN 0 3 and 2 parts HC 1 . 

Ag and Ir, melted with lead, are very difficult to alloy, the Ir 
tending to float and resist alloying. Iridium, in an alloy of Ir 
1 part, Ag 12, 15, or 18 parts, is partly soluble in strong H 2 S 0 4 
(1.84 sp. gr.), HN 0 3 (1.20 sp. gr.), and HN 0 3 (1.28 sp. gr.). 

When the ratio is Ir 1 part, Ag 3 parts, the amount dissolved 
in the same acids is still appreciable. In an alloy of Au and Ir 
the latter tends to sink to the bottom. 

Ferrous sulphate, oxalic acid, and SO2 do not precipitate 
iridium from iridic chloride. 

Palladium.—When present in a silver button, after cupella- 
tion, palladium gives the surface a raised or embossed appear- , 
ance and not a rough and pitted one like Pt. The button is 
brighter than when Pt, Ir, or both Pt and Ir are present, and 
does not stick to cupel. Alloyed with lead, some Pd is soluble 
when the alloy is treated with acetic acid. Alloyed with silver, 
it is slightly soluble in both HN 0 3 and H 2 S 0 4 , and a large excess 
of silver tends to increase the solubility. The residue after treat¬ 
ment with H 2 S 0 4 hangs together like a flocculent precipitate. 

During ordinary cupellation the loss of Pt, Ir, Rh, and Pd by 
volatilization or absorption of the cupel may be considered as 
nothing. 

There are three oxides: Pd 2 0 , the suboxide; PdO, mon¬ 
oxide; Pd 0 2 , dioxide. The last two are black. 

Osmium.—This is supposed to oxidize completely to Os 0 4 
during cupellation and to volatilize. If present in small amount, 
it may do so; if in large amount, it will not. The vapors are very 
poisonous. If Os, Ag, and Pb are placed on a cupel, owing 
to the infusibility of Os it floats on the AgPb alloy and oxidizes. 
During this oxidation, if the temperature is above that of the 
formation of PbO crystals, portions of the alloy will be thrown 
off and the cupel will be covered with small beads of Ag. If 
PbO crystals are forming, it is difficult to keep the alloy driving. 

If the Os does not completely volatilize, near the blicking- 
point black spots appear on the Ag bead, which flash off and on. 


PLATINUM AND THE PLATINUM GROUP . 


241 


but finally disappear when the button brightens. Such buttons 
are liable to sprout, appear rough on the surface, and are not so 
bright as a Ag bead. In HN 0 3 , 0 s 0 4 is formed. 

When cupelling a button containing Os do not, at the same 
time, cupel any buttons containing iridium. 


CUPELLATION OF Os, Ag, AND Pb. 


Os, 

Grammes. 

Ag, 

Grammes. 

Per Cent 
Os. 

Lead, 

Grammes. 

Parted in 

h 2 so 4 , 

Specific 

Gravity. 

Weight 

after 

Parting, 

Grammes. 

Loss, 

Grammes. 

Percent¬ 

age 

Os 

Lost. 

.00491 

. 5000 

I .OO 

6 

1.84 

.00219 

.00272 

55-4 

.00392 

. 5000 

I .OO 

20 

< i 

.OOIOO 

.00292 

74-5 

.00517 

. 2500 

2.00 

6 

i i 

.OOIOO 

.00417 

80.6 

.00565 

. 2500 

2.00 

20 

< < 

.00085 

.00480 

84.9 


The H 2 S 0 4 must be strong and thoroughly boiled to remove 
the Ag. The residue is fine and black. 

Ruthenium.—Less fusible than rhodium, but more fusible 
than osmium. 

There are three oxides: RuO, Ru 0 2 , and Ru 2 0 3 . RuO 
and Ru 0 2 are volatile, but not so easily as 0 s 0 4 . Ru 2 0 3 is 

bluish black and is formed when the metal is ignited in the 
air. 

When much Ru is present in a lead-silver alloy, if the lead 
drives, a black film soon appears and will be left on the button, 
when near the blicking point. A black scum will also be left 
on the cupel together with small silver beads, as in the case 
of Os. If a small amount is present, the button while driving 
appears more or less irregular, with spots over the surface. At 
the end it rounds up slightly, a partial play of colors will be 
noticed, it then flattens and sets. 

The surface of the silver button may be all bluish black or 
there may be black spots (Ru 0 2 or Ru 2 0 3 ) on a silvery surface. 
If only a little Ru is left, the surface is bright but rough and 
covered with bright silvery plates. 

Owing to the black oxide left on the beads, it is difficult to 
determine . the loss of Ru during cupellation, but experiments 



















242 


NOTES ON ASSAYING. 


carried out on the same line as the table under osmium seem 
to show that 20% to 45% of the Ru may volatilize. 

Iridosmium.—Experiments on this have not been satisfactory, 
owing to the difficulty of obtaining it perfectly free from other 
members of the group. 

The presence of ten per cent in a silver bead will give that 
bead an unusually bright appearance and make it look as though 
it was covered with bright, flat, silvery plates or crystals. 


METALLURGICAL LABORATORY 
EXPERIMENTS AND NOTES. 

FOURTH YEAR. 

GENERAL DIRECTIONS. 

When performing on a small scale any of the experiments 
described, it will be well for students to observe the following 
precautions: 

1. When several students are working upon the same ore, 
only one student at a time should sample it. 

2. In order to prevent resampling the ore, should anything 
happen to the experiment, always take two or more times the 
amount of ore called for in the experiment. 

3. Save the original ore sample and any products that may 
result from the work until all results are figured out and the 
report completed. 

4. Save all solutions, filtrates, concentrates, and any prod¬ 
uct relating to the test or experiment until the report is com¬ 
pleted. 

5. If any ore or product contains lumps after treatment, 
always pass it through a sieve a little coarser than the ore went 
through before treatment, then mix thoroughly and sample. 

6. Try in every possible way to avoid repeating any part of 
your work. 

Solutions.—These are made up: 

1st. By dissolving a solid in a liquid. 

2d. By adding one liquid to another. 

In these notes all solutions are made up in the following way: 

243 


244 


NOTES ON ASSAYING. 


A \% cyanide solution means that a ton of water contains 
1995 lbs. of water and 5 lbs. of KCy, or that 500 c.c. of a \% 
solution contains 498.75 c.c. of water and 1.25 grammes of KCy. 

If we wish to make up a J% solution of H 2 S 0 4 and the 
H 2 SG 4 has a sp. gr. of 1.82, we take 1990 lbs. of water and 10 
lbs. of acid, or we can take 1990 c.c. of water and 5.49 c.c. 
io.oo N 


1.82 


of acid. 


If we treat 300 grammes of ore with 70 per cent 01 water, 
3 per cent of H 2 S 0 4 , and ij per cent of bleaching-powder, we use: 


Ore. .. 
Water. 
Bleach 

h 2 so 4 . 


300 grammes 

u 


210 

4 i 


u 


n 


or 


9.0 ' 
1.82 


= 4.9 c.c., which can be measured in a small graduate. 


calcining. 

In these notes calcining means the heating of a substance out 
of contact with the air. Roasting is the heating of a substance 
with access of air. 

Examples of calcining are: 

1. Heating an ore like limonite or gothite to drive off its water. 

2. Heating limestone in a retort or crucible; the C 0 2 is driven 

off and CaO is left. ' 

3. Heating a rock to make it more porous or friable. 

In the first two cases, the substances at the end of the roast are 
different from those at the beginning, but heat alone has effected 
the change. In the last case no chemical change has taken place. 

roasting. 

We have several methods, and at the end of the roast the sub¬ 
stances with which we started have generally undergone a change. 

Oxidizing roast is where we roast with full access of air. If 
an ore is FeS 2 , the reaction will be 

2FeS 2 + nO = Fe 2 0 3 -f 4S0 2 . 








METALLURGICAL LABORATORY EXPERIMENTS. 


245 


We lose 4 parts of sulphur =128, and we gain 30 = 48, that is, 
we lose 80 parts. We start with 2FeS 2 =24o parts; therefore we 
lose 33 per cent. 

In this roast we may have Fe 3 0 4 formed, as well as Fe 2 0 3 , for 
if two roasting dishes containing FeS 2 are placed in the same 
muffle, one behind the other, the front one will receive more 
air than the back one, and the iron in the front one will probably 
exist as Fe 2 0 3 at the end of the roast, whereas in the back one 
there is liable to be considerable Fe 3 0 4 present. 

Sulphatizing Roast.—This is where we try to form sulphates. 
The S 0 2 in the oxidizing roast can further oxidize to S 0 3 , and 
this can combine with any FeO or CuO in the ore and form 
FeS 0 4 or CuS 0 4 . As 2 0 3 may also oxidize further and form 
As 2 O s , or we may have 3FeO +As 2 0 3 = Fe 3 As 2 0 6 . The Ziervogel 
Process, where the aim is to form Ag 2 S 0 4 , is a well-known example 
of this kind of roast. 

Chloridizing Roast.—In this roast salt is added to the ore and 
the object is to form sulphates as in the sulphatizing roast, some 
of which decompose the salt, thus chloridizing the ore. The salt 
may be added at the beginning, at the end, or during the roast. 
Silver ores for amalgamation or lixiviation are treated in this way. 

Roasting and Reaction Process.—This takes place in one 
kind of lead smelting. The PbS is partly roasted to form PbS 0 4 
and PbO. These then act on the PbS still unchanged and we 
have PbS + PbS 0 4 = 2 Pb + 2 S 0 2 ; 

PbS + 2 Pb 0 = 3Pb + S 0 2 . 

CHLORINATION OF GOLD ORES. 

Plattner Process of Chlorination.—This method for the extrac¬ 
tion of gold is applicable to some ores in a raw condition, but is 
especially suited for the treatment of sulphide concentrates from 
stamp-mills, the free gold having previously been extracted. 
Coarse gold is only slowly acted upon by chlorine gas. 

Experiment .—Pulverize the ore through 40-mesh sieve, sample 
carefully, grind sample through 100- or 120-mesh sieve, and assay. 

Take 300 grammes of ore (through 40), and roast dead in an 
iron pan or clay dish (6" clay dish will hold from 125 to 225 


246 


NOTES ON ASSAYING 


grammes). It is better to roast two portions of 125 than one 
portion of 250, for the ore in the latter case is apt to be too deep. 

Roast at a very low heat at first to prevent caking (the larger 
the amount of sulphides the lower the heat), and then increase to 
a high temperature, stirring frequently. 

At high temperatures iron and copper sulphate are both 
decomposed: 

2FeS0 4 = Fe 2 0 3 +S 0 2 +S 0 3 and CuS 0 4 = CuO+S 0 2 +O. 

The basic iron sulphate is only decomposed at a very high 
temperature. In a dead roast neither sulphates nor sulphides 
are present. If there is any doubt about the roast being dead, 
remove the dish from the muffle, add some fine charcoal, stir 
well, put back in the muffle and roast again, but do not stir at 
first in order to allow the charcoal to burn. Repeat this, adding 
the charcoal when the dish is outside the furnace, until no odor 
of S 0 2 is detected. 

Ores containing arsenic are especially difficult to roast dead, 
and the addition of charcoal is very beneficial. The charcoal 
reduces the sulphates to sulphides, and arseniates to arsenides, 
which are then broken up with the liberation of S 0 2 and As 2 0 3 . 
In practice it is not used in roasting, the sulphates being broken 
up by heat alone. At the completion of the roast, the gold is in 
a metallic condition and all other metals exist as oxides, with the 
exception of metals like lime, lead, and zinc, which may be present 
as sulphates. Any ferrous sulphate not decomposed would be 
oxidized by the Cl gas to ferric sulphate and would do no harm 
(6FeS0 4 +3Cl 2 = 2Fe 2 (S0 4 ) 3 +Fe 2 Cl 8 ), but the consumption of 
chemicals would of course be increased. 

Any sulphides or charcoal left in the ore would be harmful, 
as they would precipitate the gold from the AuC 1 3 : 

2AuCl 3 -f 3CuS = Au 2 S 3 +3CuC 1 2 , or 
3CuS+ 8 AuC 1 3 + i2H 2 0 = 8 Au+ 24HCIF 3CuS0 4 ; 

4AuC1 3 + 3 C + 6H 2 0 = 4 Au+ i2HCl+3C0 2 . 

Sift the ore through a 30-mesh sieve to remove any scales 
and break up any lumps. Weigh, sample, and grind sample for 
assay as fine as raw ore sample for assay. 


metallurgical laboratory experiments. 


247 


Chlorinate the remainder in the following manner: 


Mn0 2 . 3 parts or 24 gm. 

NaCl. 4 “ “32 gm. 

H 2 S0 4 . 10J “ “84 c.c. (commercial acid) 

H 2 0 . 7 “ “ 56 c.c. 



By adding the H 2 0 + H 2 S 0 4 gradually, the above will run 
about i|- hours. At the Utica Mine, Cal, the proportions used 
are 90 lbs. Mn 0 2 , 100 lbs. salt, and 200 lbs. H 2 S 0 4 . 

Reactions: 

2 NaCl+ H 2 S 0 4 = 2HCI+ Na^SO,; 

Mn0 2 +4HC1 =MnCl 2 +2H 2 0+2Cl; 

MnCl 2 + H 2 S 0 4 = MnS 0 4 + 2HCL 

Before placing the ore in bottle A, -fill it with water and see that 
it does not leak around the hole 0 and the tube x. 

Moisten the ore slightly with from 6 to 20% of water (if 
lime is present, moisten with dilute BLjSOJ and then shake iightly 
into bottle A, which it should not fill more than two thirds (200 
grammes will go nicelv in a pint jar). Dry chlorine has very 
little action upon gold. The cover ( d ) is next put on lightly to 
allow the Cl to come through; connect tubes x and z and pass 
chlorine gas through the ore for 1 to ij hours, after it is noticed 
coming from beneath cover (d). Keep a vessel of ammonia near, 
to from NH 4 C 1 . Then fill groove (e) with water, put in rubber 
gasket, clamp cover on tightly, and pass in Cl for 5 minutes 









































248 


NOTES ON ASSAYING. 


longer. Disconnect at (y), stop tube up with a piece of glass rod 
and allow jar to stand at least 96 hours, at the end of which time 
the jar should be full of gas. The gold should then be in the 
condition of AuC 1 3 . 

Most sulphides and FeS 2 are attacked by chlorine (R 2 S+ 8 C 1 + 
4H 2 0 = R 2 S 0 4 + 8 HC 1 ), while most peroxides and ferric oxide are 
not. When cover (d) is removed a strong odor of chlorine should 
he noticed. Leach the ore with as little cold water as possible to 
avoid dissolving any more foreign salts than is necessary. It can 
be done either by forcing water up through tube (x) until it rises 
above the ore, or by pouring water on the ore until it just covers it. 
Allow to stand 15 minutes and draw off at (x). Repeat three 
times more or until a portion of the filtrate tested with FeS 0 4 
gives no purple cloud (gold). If the first leaching water is pink 
it indicates the presence of manganese in the ore, which has been 
oxidized with the formation of some permanganate salt. The 
main body of the filtrate should be kept separate from the portions 
which have been tested with FeSO 4 . Save all the portions. Evap¬ 
orate the main solution to about 300 c.c. to drive off the chlorine. 
If the solution is clear, add the portions of AuC 1 3 solution containing 
FeS 0 4 and a little fresh FeS 0 4 . If it is not clear, add a few drops 
of HC 1 ; if this does not clear it or there is much residue, filter. 

The filter and contents should be saved and assayed if there 
is any likelihood of its containing gold. To the hot filtrate add 
the portions of AuC 1 3 solution containing FeS 0 4 and a little 
fresh FeS 0 4 : 

2AuC1 3 + 6 FeS 0 4 = 2Au+ Fe 2 Cl 6 + 2Fe 2 (S0 4 ) 3 . 

If the original ore contained arsenic, we might obtain here a 
whitish precipitate of ferrous arseniate, but the HC 1 should keep 
this in solution. 

Allow to stand at least forty-eight hours and then filter on a 
small filter. Save this filtrate, add a little fresh FeS 0 4 , and allow 
to stand twenty-four hours more to see whether all the gold has 
come down. The moist filter and contents are wrapped in 
10 grammes of C.P. lead, to which has been added three times 
as much C.P. silver as there is gold in the total amount of ore 
chlorinated. 

A hot cupel is brought to the front of the muffle and the lead 
and its contents dropped into it. Allow the filter to burn slowly 


METALLURGICAL LABORATORY EXPERIMENTS. 


249 


and gradually, pushing the cupel back into the muffle until the 
lead begins to drive. If it does not drive, add more lead. When 
the button is driving tip the cupel slightly in all directions, in 
order to collect any minute globules and filter-ash on the inner 
surface of the cupel. Cupel as usual, part the resulting silver and 
gold bead, and weigh the gold. 

After the extraction of the AuC 1 3 , the ore together with the 
filter is emptied from the bottle into some vessel, is dried, sifted 
from the filter of quartz (a little quartz in the ore will not matter), 
weighed, sampled, a sample put through the same mesh sieve 
as the sample of the raw ore used for assay, and valued. 

Students should obtain all the data given below and should 
hand in a report similar to the following: 

Number and character of the ore. 

Length of roast and how conducted. 

Length of time of passage of chlorine through the ore, time of 
contact with it, and whether present on opening jar. 

Manner of leaching and time. 

Precipitant used, i.e., whether FeS 0 4 , H 2 S, or some other. 

Base all the following results on the raw ore or the ore that you 
started with and the gold in it. 


In columns 3 and 4 carry out figures to four places of deci¬ 
mals; if next figure is 5 or more, increase the preceding figure 
by one. 



1 

Weight. 

2 

Assay 
per Ton. 

. 3 

Weight of 
Gold in 
Grammes. 

4 

Percentage of Gold 
Lost in Roast. 

Raw ore. 

500 gms. 

475 “ 

5.4 oz. 

5-6 “ 

. 0926 

.0912 

.0014 . 

--- = 1.51% 

.0926 0 

Roasted ore. 



Ore actually chlorinated = 381 grammes. 

Ore after leaching with water=38o grammes. Assay perton = .4 oz. 
Gold in tailings (29.166 : 380 :: .0004 : #) = .oo52 grammes. 

If we had chlorinated 475 grammes of ore, the tailings would have been 473 
grammes; therefore the gold in the tailings from 475 grammes 

(29.166 : 473 : : .0004 : rv) = .oo65 grammes. 

Gold actually recovered by chlorination from 381 grammes = .0677 grammes 
Gold recovered, based on 475 11 =.0844 “ 


. 084 4 

.0926 


91 . 14 % 

















NOTES ON ASSAYING . 


250 


The following tables should always be added: 

Table I. Table II. 

Percentage of gold lost during roast. 1.51% 1 *51% 

“ “ “ “ in tailings. . . 7.02% 7-02% 

“ “ “ saved (actually) . 91.14% (Based on tailings assay) 91.47% 

99.67% 100.00% 

On Nova Scotia concentrates it sometimes seems advisable 
to add H 2 S 0 4 to the AuC 1 3 solution in preference to HC 1 , but 
ferrous arsenate and arsenite are both more soluble in HC 1 than 
in H 2 S 0 4 . 

The gold could also be precipitated from an AuC 1 3 solution 
in any of the following ways: 

2 AuC 1 3 + 3H 2 S = Au 2 S 3 + 6 HC 1 ; 

8 AuC 1 3 + 3H 2 S+ 1 2 H 2 0 = 8Au + 24HCI+ 3H 2 S0 4 . 

Which reaction takes place depends upon whether the solution 
is hot or cold: 

2 AuC 1 3 + 3SO.J+ 6 H 2 0 = 2Au+ 6 HC 1 + 3 H 2 S 0 4 ; 

2 AuC 1 3 + 3PbS = Au 2 S 3 + 3PbCl 2 . 

Where charcoal is used as a precipitant, Mr. W. M. Davis, 
the inventor, claimed that the reaction which takes place is 
4AuCl 3 +3C+6H 2 0 = 4Au+i2HCl+3C0 2 , but it seems probable 
that the following also occur: 

6 H 2 S 0 4 + 3C = 6 H 2 0 + 6 S 0 2 + 3C0 2 ; 

6S0 2 +4AuC 1 3 + i2H 2 0 = 4Au+6H 2 S0 4 + 12HCI. 
Aluminium-foil may also be successfully used in the laboratory. 
In practice the gold and impurities thrown down by FeS 0 4 
or H 2 S are filtered, and if many impurities are present the whole 
material is either roasted at a very, very low heat, to drive off the 
arsenic and other volatile compounds, or else boiled with H 2 S 0 4 
or HC 1 to remove arsenic, iron, and any soluble substances. It 
is then dried and smelted in a graphite crucible with borax glass 
and a little soda, and towards the end of the operation a little 
nitre. 

Borax glass may be replaced by Si 0 2 , i.e., the slag must be 
acid so that iron will not go into the bullion. The slag is 
skimmed off and the gold poured into a very hot mould which 
has been oiled or into which fine rosin has been sprinkled. 




METALLURGICAL LABORATORY EXPERIMENTS. 


2 5 * 


Formerly the excess of chlorine in the AuC 1 3 solution was 
neutralized by passing S 0 2 gas into it, but it is now found 
better to neutralize with FeS 0 4 which is oxidized to the higher 
sulphate. 

Liquid chlorine, which comes in tanks holding about 130 lbs., 
is used in some works, and 1 to 2 lbs., it is said, will chlorinate 
a ton of well-roasted ore. 

The FeS 0 4 solution should be clear and light green in color. 
It can always be kept in this condition by dissolving the salt in 
FLO, filtering, and transferring the solution to a bottle. Now 
add a piece of iron and a little H 2 S 0 4 , so that hydrogen will be 
given off continually. A stopper is now placed in the bottle, 
arranged as per sketch, which allows 
the hydrogen to escape, but no air 
to enter. 

When you wish any of the 
solution do not try to pour from 
opening (c), but remove the stopper. 

Effect of Impurities upon the 
Precipitation of Gold from AuC 1 3 .— 

The following experiments were 
carried out bv Mr. A. L. Hamilton, 
class of 1 goo: 

Two solutions of AuC 1 3 were 
used, one containing, per 200 c.c. 
of solution, .01087 grammes of gold, 
the other .01120 grammes. These 
values were obtained by three 
methods, throwing down the gold 
from 200 c.c. of solution by means 
of FeS 0 4 and H 2 S and evaporating 
200 c.c. with litharge. In the 
experiments the required percent¬ 
ages of the chlorides of copper, 
lime, and magnesia were weighed, 
placed in beakers, and then dis¬ 
solved in 200 c.c. of the gold chloride solution. In the case of 



o, glass plug; b, rubber tube; 
c, slit in rubber tube; d, glass 
tube open at both ends; e y 
stopper; /, iron nail. 













NOTES ON ASSAYING. 


252 


arsenic, arseniate of soda was used and the solution was made 
acid with H 2 S 0 4 before the addition of the FeS 0 4 . 

The following table will give the results of the tests in a con¬ 
densed form: 


Per 
Cent 
of Ca 
in 

Solu¬ 

tion. 

Gold 

not 

Precip¬ 

itated, 

Per 

Cent. 

Time 

of 

Precip¬ 

itation, 

Hours. 

Per 
Cent 
of Mg 
in 

Solu¬ 

tion. 

Gold 

not 

Precip¬ 

itated, 

Per 

Cent. 

Time, 

Hours. 

Per 
Cent 
of Ar¬ 
senic. 

Gold 

not 

Thrown 

Down, 

Per 

Cent. 

Time, 

Hours. 

Per 

Cent 

of 

Copper. 

Gold 

not 

Precip¬ 

itated, 

Per 

Cent. 

.09 

.297 

115 
< < 

.012 

1.38 

120 

.017 

1.07 

3 °° 

.002 

. 276 

.18 

.644 

.029 

I.32 

< < 

•045 

2.81 

( i 

.004 

. 276 

•36 

• 5°7 

i C 

.058 

I.25 

< C 

.088 

4.64 

i i 

.oil 

.460 

.90 

6.58 

< < 

•11 7 

I. II 

< i 

.265 

9.46 

i ( 

.019 

. 276 

I.80 

6.08 

( ( 

• 234 

.67 

( i 

•443 

10.27 

i i 

•03 

. 276 

3.60 

6.00 

c i 

• 5 8 5 

.80 

i ( 

.885 

11.61 

i < 






1.17 

.98 

(( 

1.77 

12.46 

( ( 




All the above results are in each case the average of two experi¬ 
ments. In the tests where lime, magnesia, and arsenic were 
present FeS 0 4 was used as a precipitant; where copper was pres¬ 
ent H 2 S was used. 

A small amount of copper seems to have very little effect either 
when H 2 S or FeS 0 4 is used. A small amount of magnesia seems 
to be a detriment, and arsenic a great detriment. In the case 
of lime, anything above .5% appears to be harmful. To see 
whether CaS 0 4 would drag gold down from a chloride solution 
the following tests were made: 

AuC 1 3 solution. 200 c.c. 

Gold contents.01120 grammes. 

Solutions stood 250 hours. 


Grammes CaCl2. 

Percentage of Ca 
in the Solution. 

Cubic Centimetres 
of H 2 S 0 4 added. 

Gold Recovered from the 
CaSC>4. Average of 
two tests. 

. 2 

•03 

2 

.00004 grammes 

1.0 

. 18 

5 

.00005 ‘ ‘ 

2.0 

•36 

8 

.00008 ‘ ‘ 

5 

.90 

10 

.00004 ‘ ‘ 

10 

1.80 

15 

.00008 “ 

20 

3.60 

25 

.00009 ‘ ‘ 


Barrel Process of Chlorination.—Gold ores which have been 
roasted, in order to free them of sulphur, arsenic, and similar 









































METALLURGICAL LABORATORY EXPERIMENTS. 


2 53 


impurities, may be chlorinated by this process, in which the 
chlorine is generated by means of bleaching-powder and H 2 S 0 4 . 

Laboratory tests may be successfully carried out as follows: 

Experiment. —Sample and assay the original ore; if it is raw, 
roast it as described under the Plattner Process. Take a pint 
or quart fruit-jar having a glass cover and rubber gasket, and 
test it with bleach and H 2 S 0 4 to see whether it is tight. If so, 
put some water (equal in weight to at least 70% of the roasted 
ore taken for chlorination) in the jar. Now take 75 or more 
grammes of roasted ore (through a 30- or 40-mesh sieve) and mix 
it with the specified amount (see bulletin-board) of bleach, pass the 
whole through a 20-mesh sieve to remove any lumps, and pour 
the mixture into the jar. The object of this is to prevent the 
balling up of the bleach and the formation of CaS 0 4 on the out- 
side of the lumps and hence a loss of bleach, when the H 2 S 0 4 is 
added. Add the specified amount of H 2 S 0 4 and stopper the jar 
quickly and tightly by means of the glass cover and rubber gasket 
or with a solid rubber stopper and a large iron washer between the 
rubber stopper and the spring clamp. Wrap the jar in a cloth 
or towel , in case it should break, shake easily and gently, and 
then rotate in the laboratory apparatus jor the required length 
oj time. The contents of the jar should be of the consistency of 
thin mud, and the amount of water depends both upon the charac¬ 
ter of the ore and the amount of H 2 S 0 4 and bleach used. The 
amount of these depends upon the character of the ore and the 
impurities in it, which have to be converted into chlorides and 
sulphates. 

The acid is generally 1J to 2 times the weight of the bleaching- 
powder used. When the jar, vessel, or barrel has been revolved 
the required length of time it is opened, when it should smell very 
strongly oj chlorine. Fill jar with water, stir well, and allow to 
stand 15 minutes; decant solution, cover the ore with H 2 0 , allow 
to stand, again decant and then throw contents on a large filter, 
allow to drain, cover contents of filter with water, again allow to 
drain, and repeat twice more or until a concentrated solution of 
FeS 0 4 gives no test for gold when added to the filtrate and 
allowed to stand some time. 

The main body of the filtrate should be kept separate from 


254 


NOTES ON ASSAYING. 


the portions which have been tested with FeS 0 4 . Save all the 
portions. Besides the AuC 1 3 , the filtrate contains sulphates sol¬ 
uble in water and possibly some free H 2 S 0 4 . Evaporate the solu¬ 
tion to 350 or 400 c.c.j but remember that CaS 0 4 is less soluble in 
hot H 2 0 than it is in cold; therefore, if the filtrate is heated .and 
evaporated too far, CaS 0 4 will separate out. This can be dis¬ 
solved in HC 1 , but if we have HC 1 and H 2 S 0 4 both present in the 
solution, the gold will not wholly come down. 

Therefore, if CaS 0 4 does come down, we must either filter 
it off (save filter and contents) before adding the FeS 0 4 , or 
evaporate the whole solution down so far in a casserole or evapo¬ 
rating-dish that when 40 grammes of litharge and 2 grammes of 
silica are added the mass will be dry and granular. Then make 
a regular fusion of the whole residue in a crucible glazed on the 
inside. Cupel the lead button, after the addition of silver, and 
part the resulting silver and gold bead for gold. If the AuC 1 3 
solution is filtered, the filtrate is treated as per Plattner Process, 
commencing, “To the filtrate add,” etc. (page 248). 

Place the filter and contents (ore after leaching) in a roasting- 
dish and burn the filter in a muffle, pass the whole dry material 
through a 20-mesh sieve, to remove any lumps , weigh, sample, 
crush sample through same sieve as the original sample for assay, 
and value. From these data and the weight and assay of the ore 
used, calculate the percentage of gold extracted and other data as 
per example under Plattner Process. Give in jull all the data 
possible connected with the experiment , and make out a report 
similar to that under Plattner Process (pages 249 and 250). 

Of course the whole object of the experiments on any ore is 
to convert the gold into AuC 1 3 , and have the consumption of 
■chemicals small, and the time consumed in treatment as short as 
possible. A silicious ore, for instance, would consume no chlorine, 
while a calcareous ore would consume a great deal. For this 
reason a little salt is often added to the ore just previous to its 
being discharged from the roasting-furnace. In some laboratory 
tests I have found it necessary to add 6 per cent H 2 S 0 4 and 10 
per cent of bleach before a successful chlorination was obtained. 
These-high percentages would, of course, be prohibitory in practice. 

In some ores better results seem to be obtained by adding all 


METALLURGICAL LABORATORY EXPERIMENTS. 


2 55 


the acid and bleaching-powder at one time and then rotating the 
vessel; in others, by adding part of the chemicals, rotating for 
some hours, and then adding the remainder of the chemicals and 
rotating again. 

In some works water is added first, then the acid, and lastly 
the bleach; in others, the H 2 S 0 4 and water are first charged, then 
the ore, and lastly the bleach; still others add the water, bleach, 
ore, and lastly the acid. 

The water used for washing is generally 3000 to 5000 lbs. 
per ton of ore treated. 

Bleaching-powder, when fresh, contains from 25 to 37% avail¬ 
able chlorine, and it should be kept in a cool, dry place, for if 
it becomes moist it is worthless: 

CaCl 2 0 2 + CaCl 2 + 2H 2 S0 4 = 2CaS0 4 -f 2H 2 0+ 20,. 

Remsen gives these reactions: 

Ca(C 10 ) 2 + H 2 S 0 4 = CaS 0 4 + 2HCIO; 

CaCl 2 + H 2 S 0 4 = CaS 0 4 + 2HCI; 

2HCI+ 2HCIO = 2H 2 0+ 2d 2 . 

The proportion of chemicals and ore formerly used in some 
large works is as follows: 



Ore, 

Ton. 

h 2 so 4 , 

Pounds. 

Bleach, 

Pounds. 

h 2 o, 

Gallons. 

Approx¬ 
imate 
Cost per 
Ton. 

Mt. Morgan, Queensland. 

1 

33 

30 

80 

$7.50 

Haile Mine, S. Carolina. 

I 

15 

IO 

120 

4-65 

Golden Reward, Dakota. 

1 

40 

15 

100 

5 - 5 ° 

Gibbonsville, Idaho. 

I 

12 

9 

• • • • 

5.0° 

Gillette, Colorado. 

I 

30-40 

15-20 



North Brookfield, Nova Scotia. .. . 

I 

3 ° 

15 





Cost if to 2 cents 





per pound. 




T. K. Rose says that, “ at ordinary temperatures, water will 
absorb 2V3 volumes of chlorine gas and enough lime is added to 
have the solution in the barrel saturated.” 

The time of rotating the barrel or vessel varies from two to 
six hours. 






















256 


NOTES ON ASSAYING. 


The following series of tests may be tried to see if a satisfactory 
extraction can be obtained: 





Percentage of 
the Roasted Ore 


No. 

Weight of Ore 
Used. 

HgO Used. 

Taken. 

Treatment. 

Acid. 

Bleach. 





A 

300 gms. 

< < < < 

70% of ore 

1% 

1% 

Rotate to 3 hours. 

ii ii ii <i ii 

B 

<C U l * 

2 % 


C 

<< << 

4 i < i ii 

2 % 

1% 

j Add £ and rotate 1 hour. 

1 Add rest and rotate 1 hour. 

D 

ii it 

H it i < 

i% 

ii % 

Rotate 2 hours. 

E 

ii ii 

it a ii 

3 % 


j Add \ and rotate 1 hour. 

/ Add rest and rotate 1 hour. 

F 

a a 

a a a 

4 % 

2% 

Rotate 2 hours. 

G 

a a 

a t i a 

4 % 

2% 

“ 3 “ 

H 

< < Cl 

a it a 

5% 


“ 3 “ 


The smallest percentage of chemicals that will give a satis¬ 
factory extraction having been determined, the next experiments 
should be made in regard to the time necessary to give an equally 
good extraction. The shorter the time, the greater the number 
of charges per twenty-four hours, consequently the larger the 
tonnage per day and the smaller the cost of treatment per ton. 

These data having been established, the student should con¬ 
firm the smaller tests by making tests on a much larger scale. 

THE CYANIDE PROCESS FOR TREATMENT OF GOLD ORES. 

Concentrates from stamp-mills and ores high in sulphurets and 
rich in gold were formerly either smelted or else roasted and chlor¬ 
inated by some of the well-known processes, such as the Plattner 
or barrel. The cyanide process was intended, and it was origi¬ 
nally claimed for the process that it would treat ores of this char¬ 
acter, as well as mill tailings, in a raw condition, a large per¬ 
centage of the gold and a part of the silver going into solution as 
cyanides. Many ores can be treated successfully in a raw con¬ 
dition, but many others have to be roasted first. They are next 
treated with an alkali wash, if necessary, and finally with weak 
solutions of KCN. The size of the material varies from f-mesh to 
slimes, but the coarser the ore can be kept, owing to the ques¬ 
tion of leaching, with a satisfactory extraction, the better. 

























METALLURGICAL LABORATORY EXPERIMENTS. 257 

KCN exposed to air deliquesces and emits HCN as well as 
ammonia: 

2KCN + C 0 2 +H 2 0 = K2CO3 + 2HCN; 

2KCNO +4H2O = K2CO3 + (NH 4 ) 2 C 0 3 ; 

KCN + O (air) = KCNO, 

Cyanicides are substances like iron or copper which destroy 
KCN: 

Fe + 6KCN + 2 H 2 0 = K 4 FeCN 6 + 2KOH+2H. 

The KCN solutions vary from .005 to \%; weak solutions 
apparently have a greater solvent power upon gold than the 
stronger ones. Air is supposed to be a great factor in the solu¬ 
tion of the gold: 

2AU+4KCN + O +H 2 0 = 2KCN, 2 AuCN + 2KOH; 

i.e. 2X196:4X65=3:2, or 2 parts KCN should dissolve 3 parts 
of gold, but in practice 30 to 50 parts are required. The follow¬ 
ing are also said to take place: 

2AU+4KCN + 2H2O + 2O = 2 KCN, 2 AuCN + 2KOH+H2O2; 

2 Au+4KCN +H2O2 = 2 KCN, 2 AuCN + 2KOH. 

The gold may be precipitated from the auric cyanide by 
zinc shavings, by zinc dust, by charcoal,* or by electrolysis. 
When zinc is used we have 2KAuCN2+Zn = 2Au + K 2 ZnCN4, 
i.e., 1 lb. of zinc should recover 6 lbs. of gold, but in practice 
it only recovers about 1 ounce of gold. 

In practice, consumption of KCN per ton of ore is J lb. to 1J lbs. 
“ “ “ “ zinc “ “ “ “ “ Jib. “ f “ 

FeS 0 4 , oxalic acid, H 2 S, and reagents which throw down 
gold from AUCI3, do not throw it down from the auric cyanide. 
In the present tests we will not attempt to recover the gold from 
the 2 KAuCN 2 , but will base the extraction upon the assay of 
the ore before and after treatment with the KCN. In testing 


* See Mining and Metallurgy, vol. 7, 1898-99, p. 190. 



258 


NOTES ON ASSAYING. 


ores by this process, the following points would naturally have 
to be decided: 

1 st. Is the ore or material adapted to this process; free 
H 2 S 0 4 , FeS 0 4 , and copper salts not only interfere with it, but 
increase the consumption of the cyanide: 

2KCN + H 2 S 0 4 = K 2 S 0 4 + 2HCN (hydrocyanic acid); 

2KCN + FeS 0 4 = K 2 S 0 4 + FeCN 2 (ferrous cyanide); 

6KCN + FeS 0 4 = K 2 S 0 4 + K 4 FeCN 4 

or FeCy 2 +4KCN = K 4 FeCN 6 ; 
FeC03 +6KCN = K 2 C03 + K 4 FeCN6. 

2d. How coarse can the ore be kept while yet allowing a 
successful extraction (fine ores, slimes, and especially aluminous 
ores are difficult to leach and filter). 

3d. What percentage of KCN will extract the highest per¬ 
centage of gold with the smallest consumption of itself. 

4th. The proper strength of KCN having been found, what 
is the shortest length of time the ore can be left in contact with 
it, still giving you a successful extraction. 

5th. Can the extraction be improved either by agitating the 
ore and the solution or by aerating them. 

Tests can be made as follows: 

Ore treated in open beakers. 

Ore treated in closed vessels. 

Ore agitated in open or closed vessels. 

Ore acted on for a long time, solution drawn off, and then treated 
with a weaker solution. 

Ore acted on during several periods, either short or long, and 
allowed to aerate between them. 

Ore kept in agitation and aerated at the same time. 

Experiment .—Sample and assay the original ore. Take 2 A.T. 
to 500 grammes of ore (I prefer to take two portions of 2 A.T. 
each), and treat for a given length of time with a solution of KCN, 
according to the data given on the bulletin-board, an example of 
which is given on page 261. 






METALLURGICAL LABORATORY EXPERIMENTS. 


2 59 



Filter by decantation on a large filter and wash thoroughly 
with water. The filtrate is thrown away. Invert the filter and 
contents on a roasting-dish and heat first in front of the muffle, 

then within, and finally burn the filter. Weigh 
the residue, and if it is caked or in lumps, pass 
it through a sieve slightly coarser than the one 
through which the ore was originally crushed. 
Mix thoroughly and divide the ore into two 
equal portions by placing a spatula full, first on one balance-pan of 
the pulp-balance and then on the other, until they exactly balance . 
Transfer one portion, if ore is as fine as 30-mesh, without any 
further grinding, to a crucible, and then weigh the other. Make 
record of the weight. Assay both portions, which may not check 
exactly, owing to the coarseness of the ore, but this saves grind¬ 
ing, and as we obtain the total gold in the total ore it is immaterial 
whether they check or not. 

The total ore after treatment could be assayed in one crucible, 
but if the crucible was eaten through or any accident occurred, 
the test would have to be repeated; therefore the tailings are 
divided into two equal portions and assayed separately. 

From these data and the assay of the original ore, calculate 
the percentage of gold extracted. The ore to be tested, the 
experiments to be made, and the data to be obtained, will be posted 
upon the bulletin-board. 

Report .—The report upon this process should be handed in 
with the following data and table: 

Number of the sample or name of the ore. 

Character of the ore. 

Size of the ore, i.e., what sieve it will pass through and whether 
it is raw or roasted. 

Assay of the ore. 

Amount of solution and the percentage of KCN in the solution 
used upon the ore. 

Period of contact of this solution with the ore. 

Conditions under which it acted; i.e., was an open or closed 
vessel used; were the ore and solution agitated; was the ore 

aerated ? 






260 


NOTES ON ASSAYING. 


Diameter of vessel used and depth of ore and solution in the 
vessel. 


Ore 

Used 

2 A.T. 

Total 
Gold in 
Ore Used, 
Gms. 

Weight of 
Ore after 
KCy 
Treat¬ 
ment. 
Gms. 

Assay of 
Ore after 
Treat¬ 
ment. 
Gms. 

Total 
Gold 
in Tail¬ 
ings 
(a), 
Gms. 

Total Gold in 
Tailings (£>),* 
Gms. 

Extract¬ 

ed, 

Based 
on (a). 

Extract¬ 

ed, 

Based 
on ( b ). 

Example 
No. 1. 
Assay 
per ton. 
1.36 oz. 

.00136 

2 

28.915 

.00008 





.00272 

28.900 

.00008 

.00016 

.000161 

94 - 1 % 

94 % 

Example 
No. 2. 

.03820 

27.140 
27.140 

r^. t'- 
O O 

O O 

O O 

.00140 

.00150 

96 - 33 % 

96.07% 


The KCy solution from the last test was evaporated down with PbO (30 
grammes) and the residue fused in a glazed crucible. 

Gold = 0.364 grammes, or an extraction of 95.28%. 


Treatment of Ores by Potassium Cyanide.—In testing a raw 
ore to see whether it is amenable to this treatment, I prefer to 
use fine ore—that is, ore passing a 30- or 40-mesh sieve at least— 
and treat it with a strong solution—that is, \% to \% solution—for 
a long period of time, 196 hours or more. This will give some 
idea of its amenability. If the results are negative then make 
the same tests on some ore which has been roasted. If these give 
negative results, then some modification of the process, such as 
aeration, agitation, or percolation, will have to be tried. 

If the tests on the raw ore (through 30- or 40-mesh) give 
satisfactory results, then of course roasting is unnecessary, but it is 
necessary to determine the best strength of KCN, how coarse it 
is possible to keep the ore and the shortest and best method of 
contact. To do this, take the same ore, keep time of treat- 


* There is probably a mechanical loss of tailings, and these tailings are 
assumed to assay the same as those saved. 
























METALLURGICAL LABORATORY EXPERIMENTS 


261 


ment the same (196‘hours), and decrease the percentage of KCN 
in the solution until the extraction begins to decrease. Having 
determined the weakest solution that can be used to advantage, 
keep the time of contact the same and experiment with coarser 
ore until size limit of ore is reached. 

Next make tests and see whether it is not possible to shorten 
time of contact by means of a series of alternate contacts and 
aerations or by aeration alone. Agitation may also be tried and 
experiments carried on in regard to bromo-cyanogen. In the 
treatment of some ores the first KCN solution is a strong one 
followed by a weaker one; in other ores the treatment is just 
the reverse. 

When the small tests give satisfactory results they should be 
confirmed by tests on a larger scale and the consumption of 
KCN carefully determined. This last is especially important, 
lor, owing to the poor quality of air in laboratories, tests made 
there usually show a much higher consumption than those made 
in a mill or in places where the surrounding air is purer. 


EXPERIMENTAL TESTS ON A GOLD ORE AS TO ITS ADAPTABILITY 

FOR THE CYANIDE PROCESS. 


Silicious ore carrying i* per cent FeS 2 . Raw, crushed through 20-mesh sieve. 
Assay, .38 oz. Value @ $2o fl7 / 100 per oz. = $7 85 / 100 . 


No. of Tests 

Weight 
of Ore 
Taken. 

Water 

Used. 

Per Cent 
of KCy 
in Solu¬ 
tion. 

Time of 
Contact, 
Hours. 

Extrac¬ 

tion, 

Per 

Cent. 

A 

2 A.T. 

60 c.c. 

i 

16 

65 

B 

< i 

< « 

u 

16 

66* 

C 

1 1 

tt 

a 

48 

82 

D 

* t 

t ( 

u 

48 

59 

E 

11 

tt 

i 

48 

76* 

F 

a 

t ( 

u 

120 

96 

G 

*t 

it 

ii 

120 

89 

H 

tt 

tt 

ii 

140 

94 

I 

n 

tt 

ii 

90 

95 


Washed with water and then 
with a weak solution of 
KOH and again with water, 
before treatment with KCy. 
Did not wash. 

( i i i i i 

Roasted the ore and then 
washed with KOH and H 2 0 . 
Raw ore; did not wash. 

i i ii i i i i ii 

Treated in an inverted bottle. 
























262 


NOTES ON ASSAYING. 





Oro 
Filtei 

Filter plate 


In the last test the ore is first kept in contact with the KCN 

solution, then allowed to aerate (time of contact 
40 hours, time of aeration 10 hours), again placed 
in contact, and then washed with water. 

The above tests, with the exception of I, 
were made in open beakers, and the ore was 
not stirred or agitated. They indicate that 
this ore can be treated successfully by this 
process, that a \% solution of KCN seems better than £%, 
and that roasting and aerating both increase the extraction. 
Further tests should be made in regard to the time of contact, and 
tests on large amounts of ore should be made to see whether they 
confirm the small ones, and also in regard to the consumption of 
KCN. 


—Glass tube 
Clamp 


THE CYANIDE PROCESS AS APPLIED TO THE CONCENTRATES 
FROM A NOVA SCOTIA GOLD ORE* 

The following work, performed by Mr. W. A. Tucker, of the 
class of 1893, in the Mining Department of the Massachusetts 
Institute of Technology, seems to me to be worthy of publication. 
I believe it has always been considered that the presence of arsenic 
especially interferes with the extraction of gold by the cyanide 
method. Mr. Tucker’s work, although made on a laboratory 
scale, certainly seems to disprove this view, and to show that, 
even with a very large percentage of arsenic present in the ore,, 
a high extraction may be obtained without an excessive con¬ 
sumption of KCN. 

The ore from which the concentrates were obtained was a 
gray argillaceous schist and slate, with stringers and veins of 
quartz running through it. It carried free gold and about 12 per 
cent of sulphides. The ore was crushed with stamps; the free 
gold was collected in the ordinary way on silver-amalgamated 
copper plates; and the sulphides, which consisted chiefly of 


* Transactions of the American Institute of Mining Engineers. Florida Meet¬ 
ing, March, 1895 












METALLURGICAL LABORATORY EXPERIMENTS . 263 

arsenopyrite and pyrite, with very small amounts of galena and 
chalcopyrite, were concentrated and collected by means of a 
Frue vanner. 

A carefully taken sample gave: 


Gold. 6.17 ounces per ton 

Arsenic.30.6 per cent 


The latter figure would correspond to about 66.5 per cent of 
arsenopyrite in the concentrates. 

The work to be done was outlined as follows: 

1. Sizing the concentrates; 

2. Assaying the different sizings; 

3. Treating these different sizings with KCN of different 
degrees of strength for different periods of time. 

Owing partly to the small amount of concentrates found on the 
40- and 60-mesh sieves, and partly to the lack of time, the follow¬ 
ing four series were substituted in place of carrying out No. 3: 

Series I .—Treating concentrates with a given amount of a 
1 per cent KCN solution for different periods of time. The solu¬ 
tion, instead of being all added at once, was added in three portions. 

Series II .—Treating a given amount of concentrates with an 
equal quantity of a 1 per cent solution of KCN for different 
periods of time, the KCN not being renewed as in Series I. 

Series III .—The same as Series II, except that the con¬ 
centrates were revolved with the KCN solution in bottles, and 
did not simply stand in contact with it, as in the previous series. 

Series IV .—Concentrates and solution in motion; strength 
of KCN solution, time of contact and amount of solution varying. 

SIZING AND ASSAYING CONCENTRATES. 

A sample of the concentrates sized and assayed resulted as 
follows: 




264 


NOTES ON ASSAYING. 


Sieve 

Through. 

Mesh. 

On. 

Proportion of 
Sample. 
(Per Cent.) 

Assay. 
(Ounces 
per Ton.) 

Gold. 

Proportion of 
Total Gold. 
(Per Cent.) 


40 

O.412 

25.70 

0.000363 

1 • 7 1 

40 

60 

O.449 

27. IO 

0.000417 

1.96 

60 

80 

4.OIO 

10.69 

0.001468 

6.90 

80 


92.710 

6.00 

0.019005 

89-43 

Lost. 


2.419 




'T' ntnl 


IOO.OOO 


0.021253 

100.00 






The above assays include, of course, the free gold (pellets) 
which may have been found on the 40-, 60-, and 80-mesh sieves. 


TREATMENT WITH CYANIDE. 

Series I .—One A.T., or 29.166 grammes, of concentrates 
passed through a 30-mesh sieve, and assaying 6.17 ounces per 
ton, was treated with 100 c.c. of KCN (1 per cent) solution. This 
solution was added at three different times in equal portions, 
the first portion being drawn off before the second was added, 
and so on. 

The apparatus employed was an inverted glass bottle, with 
the bottom cut off, and a perforated porcelain plate laid across 
at the point of contraction to the neck, so as to form (in the in¬ 
verted position) a false bottom. Below this, the neck was closed 
with a rubber stopper, through which passed a glass tube, fitting 
outside to a rubber tube, closed with a pinch-cock. 

The result of these tests was as follows: 


Time of A 
Calculated 
Add 

Second. 

(Hours.) 

dding KCN. 
from First 
ition. 

Third. 

(Hours.) 

Time of 
Withdraw¬ 
ing the Third 
Addition of 
KCN. 

(Hours.) 

Assay of 
Tailings in 
Ounces per 
Ton. 

Percentage 
of Gold 
Extracted. 

Grammes of 

Consumed. 

Grammes of 
KCN Used 
per Gramme 
of Gold 
Extracted. 

i6i 

20^ 1 

23 

1.92 

68.88 

0.112 

26.4 

i6i 

20^ 

23 

2-39 

61.26 

0.124 

32.8 

2 3 h 

30 

51 

i -43 

76.82 

0.168 

35-4 

22% 

29 ^ 

51 

2.82 

54-29 

0.146 

43-6 

23 

65 

70 

1.44 

76.66 

°-i 55 

32.8 

23 

6 5 

70 

2.78 

54-94 

0.156 

46.0 

24 

44 

94 

I - 5 I 

75-52 

0.169 

36.3 

24 

44 

94 

' 2.67 

56.72 

0.206 

58-9 

24 

44 

94 

i- 5 i 

75-52 

0.165 

35-4 

20^ 

94 i 

118 

1.62 

73-74 

0.164 

36.0 

20^ 

94 l 

118 

1.56 

74.72 

0.202 

43-8 

20^ 

94 i 

118 

1.23 

80.06 

0.202 

40.9 








































METALLURGICAL LABORATORY EXPERIMENTS. 265 

These results were not at all satisfactory, for they neither indi¬ 
cated that the extraction increased with the time of contact, nor 
did they show in what period of the contact the solution of the 
gold took place. 

Series II .—The apparatus used was the same as in Series I. 
Quantity of concentrates (through 30-mesh), 25 grammes; assay, 
6.17 ounces per ton; quantity of KCN (1 per cent) solution, 25 c.c. 
The solution was not changed. 

These experiments seem to show that the extraction of gold 
increases with the time of contact of the KCN. Apparently, the 
consumption of KCN increases with the time. This large con¬ 
sumption in both Series I and II is no doubt due to the free 
access of air to the apparatus in which the tests were made. 

While working on these experiments, 25 grammes of concen¬ 
trates and 25 c.c. of KCN solution were put in a bottle, tightly 
stoppered, which was caused to revolve. The extraction was 
such an improvement on all the previous work that all other 
experiments (Series III and IV) were conducted in this way. 


Duration 
of Treatment. 
(Hours.) 

Assay of Tailings. 
(Ounces per 
Ton.) 

Per Cent, of 
Gold Extracted. 

Grammes of 
KCN Consumed. 

Grammes KCN 
Used per Gramme 
of Gold Extracted. 

16 

2.97 

5 1 -84 

.064 

23-4 

16 

2-75 

55-43 

.065 

22.2 

22 

2.30 

62.72 

.058 

i 7-5 

22 

2.17 

64.83 

.058 

16.9 

25! 

2.12 

65.64 

.065 

19.6 

71 

2.51 

59-32 

. 124 

39-5 

71 

2.24 

63.69 

.118 

35 -o 

71 

2-57 

58.35 

. 127 

41.1 

Il8 

I . 6l 

73 - 9 i 

.091 

23-3 

Il8 

I.40 

77 - 3 i 

•! 3 I 

32.5 

Il8 

I . 26 

79 - 5 8 

.134 

3 1 • 1 


Series III .—Quantity of concentrates (through 30-mesh), 25 
grammes; assay, 6.17 ounces per ton; quantity of KCN (1 per 
cent) solution, 25 c.c. Bottles and contents revolved. 













266 


NOTES ON ASSAYING. 


Duration of 
Revolution. 
(Hours.) 

Assay of Tailings. 
(Ounces per 
Ton.) 

Per Cent Ex¬ 
tracted,Calculated 
from No. 2. 

Grammes of 
KCN Consumed. 

Grammes of KCN 
Used per Gramme 
of Gold Extracted. 

2 

0-39 

93.68 

0.022 

4.44 

2 

1. II 

82.01 

0.033 

7.60 

2 

0.82 

86.71 

0-035 

7-6 3 

2 

0.82 

86.71 

0.032 

6.97 

4 

0.66 

89.30 

0.072 

15-25 

4 

0.62 

89-95 

0.042 

8.82 

Si 

0.58 

90.60 

0-053 

11.06 

Si 

0.47 

92.38 

0.063 

13.07 

23 

0.42 

93-19 

0.031 

6.42 

23 

0.58 

90.60 

O.043 

8.98 


Tnese experiments seem to indicate that to revolve the bottles 
about six hours was sufficient, and that the extra amount of gold 
extracted would hardly compensate for a longer revolution. 

Series IV .—Bottles and contents revolved. 

The large consumption of KCN in the last two tests was due 
to insufficient washing. 

In none of the tests were the concentrates washed with water 
previous to their treatment with cyanide. Owing to lack of time, 
Mr. Tucker was unable to test the solutions for arsenic, or to 


Concentrates through 30-MESH; Assay, 6.17 Ounces per Ton. 


Duration 
of Revo¬ 
lution. 
(Hours.) 

Weight 
of Ore. 
(Grms.) 

Strength 
of KCN. 
(Per 
Cent.) 

Quantity 
of KCN 
Solution, 
(c.c.) 

Assay of 
Tailings. 
(Oz. per 
Ton.) 

Per Cent 
of Gold 
Extracted. 

Grammes 
of KCN 
Consumed. 

Grammes 
of KCN per 
Gramme of 
Gold Ex¬ 
tracted. 

4 

50 

1.0 

25 

0 

00 

85.90 

.026 

2.84 

4 

5 ° 

1.0 

25 

O. 71 

88.49 

.094 

10.03 

4 

25 

0-5 

25 

0.47 

92.38 

.014 

2.86 

4 

25 

o-5 

25 

0.66 

89.30 

.009 

1.91 

4 

25 

o -5 

25 

0.89 

85-57 

.014 

3-09 

16J 

25 

o-5 

25 

o -97 

84.28 

•055 

12.33 

i6£ 

25 

°-5 

25 

o -57 

90.76 

.050 

10.42 

23 

25 

°-5 

25 

0.51 

9 i -73 

•045 

9.28 


Concentrates through 8o-mesh; Assay, 6 Ounces per Ton. 


Duration 
of Revolu¬ 
tion. 
(Hours.) 

Weight 
of Ore. 
(Grms.) 

Strength 
of KCN. 
(Per 
Cent.) 

Quantity 
of KCN 
Solution, 
(c.c.) 

Assay of 
Tailings. 
(Oz. per 
Ton.) 

Per Cent 
of Gold 
Extracted. 

Grammes 
of KCN 
Consumed. 

Grammes 
of KCN per 
Gramme 
of Gold 
Extracted. 

6| 

61 

1000 

IOOO 

1.0 

0-5 

1000 

1000 

0.70 

1.00 

88.30 

83-33 

7-033 

3 - 6 i 7 

77.06 

40.22 

















































METALLURGICAL LABORATORY EXPERIMENTS. 


267 


see whether all the gold could be recovered from them; so we 
are unable to give any data on these points. While we realize 
that these are simply laboratory experiments, that the tailings 
are in all cases too rich to be thrown away, still we consider the 
extraction remarkably high on material carrying the percentage 
of arsenic that this does. Making the tests in a closed vessel 
lessens the consumption of KCy, as one would expect. As the 
extraction also increases, this would seem to be contrary to 
Eisner’s equation, to which oxygen is necessary; and certainly 
there could hardly be enough in a small bottle to influence the 
extraction. Keeping the ore and solution in agitation certainly 
seems to have helped the extraction, although this method of 
working has met with very little success in actual practice. 

REACTIONS IN THE CYANIDE PROCESS. 

The following are some of the simpler reactions which take 
place in various stages of the process: 

In .Preliminary Treatment. 

Fe 2 0 3 ,3S0 3 + 6NaOH = Fe 2 H 6 0 6 -f 3Na 2 S0 4 . 

Fe 2 0 3 ,2S0 3 + 4NaOH+ H 2 0 = Fe 2 H 6 0 6 + 2Na 2 S0 4 . 

FeS 0 4 + CaH 2 0 2 = FeH 2 0 2 + CaS 0 4 . 

ZnS 0 4 + 2NaOH = ZnH 2 0 2 + Na 2 S 0 4 . 

In Solution Tanks. 

4KCy+ 2 Au+ H 2 0 + O = 2KAuCy 2 + 2KOH. 

4KCy+ 2Ag+H 2 0 + 0 = 2KCy,2AgCy + 2K0H. 

2 KCy+ FeS 0 4 = FeCy 2 + K 2 S 0 4 . 

F eCy 2 + 4KCy = K 4 F eCy 6 . 

Fe 2 (S 0 4 ) 3 + 6KCy+ 6 H 2 0 = Fe 2 H 6 0 6 + 3K 2 S0 4 + 6HCy. 

3 K 2 ZnCy 4 +4Au+ 20 = 4KAuCy 2 + K 2 Zn 0 2 + 2 ZnCy 2 . 

A 1 2 (S 0 4 ) 3 + 3H 2 0 + 6KCy = A1 2 0 3 + 3 K 2 S 0 4 + 6HCy. 

In Air. 

2 KCy+ C 0 2 + H 2 0 = 2 HCy+ K 2 C 0 3 . 

KCy+ 0 = KCyO. 2KCNO + 3O = K 2 C 0 3 +C 0 2 + 2N. 

2KCy+ 2H 2 0 = 2HCy+ 2KOH. 


268 


NOTES ON ASSAYING . 


Substances in Solution. 

ZnCy 2 + 4KOH( T °° lk “" ch ) =K 2 0 ,Zn 0 + 2KCy+ 2 H 2 0 . 

K 2 ZnCy 4 + CaH 2 0 2 = ZnH 2 0 2 + 2 KCy+ CaCy 2 . 

In Precipitating-tanks and Zinc Boxes . 

2KAuCy 2 + Zn = K 2 ZnCy 4 + 2 Au 
2H 2 0 + Zn = ZnH 2 0 2 + 2H 

ZnH 2 0 2 + 2 KCy = ZnCy 2 + 2KOH 

Zn+Cy 2 = ZnCy 2 

ZnCy 2 + 2KCy = K 2 ZnCy 4 

2 koh+co 2 =k 2 co 3 +h 2 o 

HCy+ KOH = KCy+ H 2 0 

Electrolysis will give from KAuCy 2 = Au+K+2Cy: 

k+h 2 o=koh 

2K0H+Zn = ZnK 2 0 2 + 2H. 

Testing a Roasted Ore for Sulphates.—The following test will 
determine if a natural ore or one that has been roasted contains 
salts, soluble in water, which would be detrimental to the Cyanide 
Process: 

Take 100 to 300 grammes of the ore and put in some vessel, 
add 300 c.c. of water, stir for 5 minutes or so and filter. Add 
slowly a small amount of KCy solution of the same strength to 
be used in leaching the ore. Watch the solution carefully for any 
cloudiness. If none appears the ore is ready to be treated or is 
probably dead roasted. If a brown color shows, soluble salts 
of iron are present in the ore and will cause a high consumption of 
KCy and precipitation of ferrocyanide compounds in the zinc- 
boxes : 

FeS0 4 +2KCy=K 2 S0 4 +FeCy 2 (ferrous cyanide). 

If a blue coloration occurs, followed by a blue or greenish 
precipitate, then the ore contains a large amount of sulphates 
and a very high consumption of KCy will take place: 

FeCy 2 +4KCy = K 4 FeCy 6 (potassium ferrocyanide). 

Alkali or Alkali Wash.—Many ores are quite acid, especially 
those that have been much weathered. This acidity is generally 


METALLURGICAL LABORATORY EXPERIMENTS. 269 

due to the presence of sulphates, especially FeS 0 4 . To determine 
the amount of lime or alkali to add to an ore in order to neutralize 
this acidity proceed as follows: 

Dissolve 10 grammes of NaOH in 1000 c.c. of water. each 
cubic centimeter contains .01 grammes of NaOH. 

Take 200 grammes of ore and leach thoroughly with water. 
Titrate this solution with NaOH. 

If we run in 26.2 c.c. of NaOH solution, then it takes .262 
grammes of NaOH to neutralize the acidity of 200 grammes of 
ore or 2.62 lbs. to neutralize 2000 lbs. of ore. 

Poisoning.—Potassium cyanide is a deadly poison and very 
quick in its action; therefore when large amounts of solution con¬ 
taining KCy are being discharged from mills, the matter is a 
serious one. Hydrocyanic acid acts directly on the nervous 
system, causing instant paralysis (jaws close and it is necessary 
to use force in opening them); hence any treatment that will 
excite the action of the nerves, such as the application of cold 
water to the spine and inhalation of ammonia, may be tried in 
cases of faintness produced by breathing the vapor of the acid. 
When KCy gets into cuts it may produce painful sores, and men 
employed in melting the zinc slimes are subject to an eruption on 
the arms and often complain of headache and giddiness. Ferro- 
cyanide of potash has been recommended for the eruption. For 
cases of internal poisoning freshly precipitated carbonate of iron 
given immediately is a good antidote (FeC 0 3 + 6KCy = K 4 FeCy 6 
+ K>C 0 3 ), potassium ferrocyanide being formed in the stomach. 
Next give a purgative. Walk person about and prevent sleep. 

The FeC 0 3 is obtained by adding a solution of carbonate of 
soda to ferrous sulphate (FeS 0 4 + Na 2 C 0 3 = FeC 0 3 + Na 2 S 0 4 ), 
and a bottle of each of these should be kept constantly on hand 
in every place where KCy is being used. The freshly precipi¬ 
tated white carbonate of iron should be given immediately, for it 
soon oxidizes to the brownish-black ferrous hydrate, and this to 

the ferric hydrate: 

FeC 0 3 +H 2 0 = Fe(H 0 ) 2 + C 0 2 ; 

2 FeH 2 0 2 +H 2 0 + 0 = Fe 2 H 6 O c . 

Dr. C. J. Martin and Mr. R. A. O’Brien, Proceedings oi 


270 


NOTES ON ASSAYING. 


Society of Chem. Industry of Victoria, Vol. I, pp. 119-129, have 
experimented on rabbits with many antidotes. They recom¬ 
mend ferrous sulphate and potash together with powdered mag¬ 
nesium oxide, which must be given as soon as possible and within 
5 minutes of the time of taking the cyanide. They advise keep¬ 
ing on hand: 

1. 30 c.c. (1 oz.) of a 23 per cent solution of ferrous sulphate. 

2. 30 c.c. (1 oz.) “ “ 5 “ “ “ “ caustic potash. 

3. 2 grammes (30 grains) powdered magnesia. 

4. A vessel to mix them in. 

5. Stomach-pump. 

Nos. 1 and 2 should be kept in hermetically sealed tubes 
which can be easily broken. Break, pour contents in the vessel, 
add the magnesia, a pint of water, shake well, and administer. ” 

Some cases of poisoning occurring when men are treating the 
zinc-box residues have been found to be due to arsenic and not 
to KCN. 

If an ore contains arsenic, some of this metal will be deposited 
on the zinc. When the zinc is treated with acid later on, the 
deadly arseniuretted hydrogen will be given off: 

Zn 3 As 2 + 3H 2 S0 4 = 2AsH 3 + 3ZnS0 4 
or 

6Zn+ As 2 0 3 + 6 H 2 S 0 4 = 2AsH 3 + 6 ZnS 0 4 + 3H 2 0. 

Potassium Cyanide.—Practically pure KCN can be purchased, 
which should be used to standardize the silver nitrate solutions. 

The ordinary commercial KCN is far from pure, as will be 
seen from the following analyses: 



No. 1. 

No. 2. 

No. 3. 

No. 4. 

No. s. 

Potassium. 

38-5% 

54 - 2 % 

18% 

21.8% 


Sodium. 

10.4 

none 

29-3 

27.1 


CN. 

38.2 

18.2 

14.4 

19.0 

13.6% 

rn /Calculated from-, 
'-'"'s ' carbonates ) 

1.0 

12.2 

8.7 

21.4 


Caustic alkali. 

3-2 

2-3 

7-9 

1.8 


Cl. 

none 

none 

14.2 

8-3 


CNO. 

none 

11.6 

none 

none 



In KCN there is 40% CN; therefore the per cent purity of 
the above lots would be 


38.2 



18.2 



36 %, 47 i%, 



40 


40 


























METALLURGICAL LABORATORY EXPERIMENTS. 


271 

Titration of the KCy Solution.—Take an aliquot part of the 
KCy solution, add a few drops of a 5% solution of KI as an 
indicator and run in from a burette a standard solution of neutral 
AgN 0 3 . The end-point is a pale bluish coloration with a slight 
precipitate of AgCy. Double cyanides of metals and potassium 
do not interfere with the titration. Silver cyanide is insoluble 
in w r ater, but is soluble in KCy and CaCy 2 ; therefore, while 
these are present, the AgCy thrown down is immediately dis¬ 
solved; when they are neutralized, the AgCy will remain as a 
precipitate. 

Any KCyO present in the solution will also give a precipitate 
until neutralized. 

The reactions which take place are as follows: 

1 a. KCy+AgN 0 3 = AgCy+KNOg. 

ib. AgCy + KCy = KAgCy 2 . 

2. KAgCy 2 +AgN 0 3 = KN0 3 +2AgCy (end-point: white pre¬ 
cipitate). 

3. AgN 0 3 +KI (indicator) = KN 0 3 +Agl when no KCy is 
present. 

If in equation (2) we continue to add AgN 0 3 , after the white 
precipitate first appears, a precipitate will continue to come 
down until all the KAgCy 2 in the solution is broken up. 

If too much alkali has been used as a wash, we may 
obtain CaCy 2 in this way: K 2 ZnCy 4 +CaH 2 0 2 = ZnH 2 0 2 -f 
2KCy+CaCy 2 . 

From the reaction AgN 0 3 + 2KCy = KAgCy 2 +KN 0 3 we see 
that 1 part AgN 0 3 is equivalent to 2 parts KCy, or 170 parts 
AgN 0 3 can be added to 130.2 parts KCy before a precipitate 
comes down. 

.*. ic.c. KCy = ^^ or 1.3056 AgN 0 3 . 

If, therefore, we add 1.3056 grammes AgN 0 3 to a litre of 
water, it will correspond to 1 gramme KCy, and each c.c. will 
contain .001305 grammes of AgN 0 3 and correspond to .001 

grammes KCy. 

Suppose that 50 c.c. of some unknown KCy solution takes 


1 



272 


NOTES ON ASSAYING. 


50 c.c. or .06525 AgN 0 3 to neutralize it, then we know that this 

corresponds to .050 KCy, or the solution contains 1 ^ = .i%KCy. 

5 ^ 

TREATMENT OF ROASTED GOLD ORES BY MEANS OF 

BROMINE* 

Mr. H. R. Batcheller, of the class of 1894, Massachusetts Insti¬ 
tute of Technology, while experimenting with chlorine gas on a cer¬ 
tain lot of roasted concentrates, met with the following difficulties: 
1. A poor extraction of the gold. 2. A very large consumption of 
chlorine gas. 3. Inability to precipitate all of the gold from the solu¬ 
tion containing the AuC 1 3 . 4. The bullion obtained was very base. 

These difficulties were the same whether the chlorine was 
generated from H 2 S 0 4 , Mn 0 2 , and salt, or whether H 2 S 0 4 and 
bleaching-powder were used. They may be accounted for partly 
by the presence of some arsenic left in the roasted ore, and partly 
by the presence of copper in the solution containing the AuC 1 3 . 

It was therefore suggested to try the effect of bromine on a 
similar lot of ore. The use of this element is, of course, nothing 
new, but in the following experiments it seemed to present many 
advantages over chlorine. 

The material worked upon consisted of some concentrates 
containing 2.31 ounces of gold per ton and 34.26 per cent of 
arsenic, which would correspond to about 74.4 per cent of arseno- 
pyrite. Considerable pyrite and a small amount of galena and 
chalcopyrite were also present. 


The material when sized and assayed showed: 



Per Cent. 

Ounces Gold per 
Ton. 

On 24-mesh sieve. 

•7 

1.9 

3 - 5 

6.0 

4 - 5 

11.0 

26.0 

45 

1.4 

j- Assaying 1.4 

| Assaying 1.2 

Assaying 1.12 
Assaying i. 19 
Assaying 1.4 

On 30-mesh sieve. 

On 40-mesh sieve. 

On 50-mesh sieve... 

On 60-mesh sieve. 

On 80-mesh sieve. 

On 100-mesh sieve. 

Through ioo-mesh sieve. 

Loss. 


100.00 


* Transactions of the American Institute of Mining Engineers. Florida 
Meeting, March, 1895. 





















METALLURGICAL LABORATORY EXPERIMENTS. 


2 73 


The line of treatment was as follows: 

1. Roasting the concentrates in a reverberatory furnace. 

2. Submitting the roasted ore to bromination in strong pre¬ 
serve-jars, “Lightning” brand, with double gaskets, the jars and 
their contents being revolved during the experiment. 

3. Precipitation of the gold by means of H 2 S: 

Roast I.—Time, five hours. 



Kilos. 

Assay, 

Ounces Gold. 

Raw ore. 

IO 

6 

Per Cent. 

40 

2. ^1 

3-36 

Per Cent. 

12.7 

Roasted ore. 

Loss. 



BROMINATION. 

Roasted ore. 500 grammes 

Bromine. 14.5 c.c. 

Water. 500 c.c. 

Time. 5^ hours 

Assay of tailings from two tests gave 0.30 and 0.32 ounces of 

gold. Based on the roasted ore, this would be an extraction of 

90.7 per cent. 

Roast II.—Time, eight hours. 



Kilos. 

Assay, 

Ounces Gold. 

"R aw nre. 

15 

8 

Per Cent. 
46.67 

2.31 

4.29 

Per Cent. 

1 

T?na<?ted ore. 

Lo< 5 S. ...r, . 



The following experiments were made to determine the proper 
amount of bromine for 500 grammes of ore: 


Roasted Ore, 
Grammes. 

Bromine, c.c. 

Time, Hours. 

Water, c.c. 

Extraction, based on 
Assay of Tailings, 
Per Cent. 

5 00 . 

3 -° 

5 * 

5 00 

90.67 

5 00 . 

3 -o 

Sh 

5°° 

89.27 

5 00 . 

i-5 

Sh 

5 00 

92-54 

5 00 . 

1.0 

5 * 

5 00 

8 l .35 

5 00 . 

°-5 

5 * 

5 00 

62.23 

5 °°. 

0.3 

5 * 

500 

60.00 














































274 


NOTES ON ASSAYING . 


The following were made to determine the shortest period of 
contact of ore and bromine giving a good extraction: 


Roasted Ore, 
Grammes. 

Bromine, c.c. 

Time, Hours. 

Water, c.c. 

Extraction, based on 
Tailings, Per Cent. 

500 . 

1-5 

5 ? 

5 00 

9 2 • 54 

5 00 . 

i -5 

4 i 

50° 

88.00 

5 00 . 

i -5 

& 

S' 00 

86.00 

5 00 . 

i -5 

2 

5 00 

8 i- 3 S 

5 00 . 

i -5 

1 

5 00 

72.02 


These tests seem to indicate that 1.5 c.c. of bromine, added to 
500 grammes of ore in 500 c.c. of water, would effect in five and 
one half hours an extraction of over 90 per cent of the gold in 
the ore. 

To test these conclusions, a third roast was made: 


Roast III.—Time, eight hours. (Ore cooled in furnace.) 



Kilos. 

Assay, 

Ounces. 

Arsenic, 

Per Cent. 

Sulphur, 

Per Cent. 

Raw ore. 

70 

2.31 

34.26 


Roasted ore. 

43-7 

3-58 

O. II 

0-34 


Per Cent. 

Per Cent. 



Loss. 

37 - 6 

3-3 

99.67 



Of this roasted ore, 15 kilos were treated with 45 c.c. of 
bromine in 15 kilos of water for four and one half hours in a 
revolving keg. The tailings showed an extraction of 85.5 per 
cent. 

As an excess of bromine was present when the keg was opened, 
at the end of four and one half hours, a second experiment was 
tried with ore, 15 kilos; bromine, 35 c.c.; time, five and one half 
hours; water, 15 kilos. 

This showed an extraction of 92.18 per cent, based on the 
assay of the tailings. The actual gold recovered from the solu¬ 
tion was only about 80 per cent, which may be accounted for 
by the presence of considerable copper in the solution. 

The expulsion of the bromine from the solution seemed to be 
best brought about by means of S 0 2 . Air and steam were both 
tried, but with poor success. After the passage of S 0 2 the solu- 
































METALLURGICAL LABORATORY EXPERIMENTS. 


2 75 


tion was quite clear, although some gold would be precipitated 
on standing. 

When the ore was chlorinated the solution at this point, con¬ 
taining the AuC 1 3 , would be quite turbid, and evidently con¬ 
tained a large amount of base metals as chlorides. These would 
necessarily interfere with the complete precipitation of the gold, 
besides making the bullion base. Some base metals, such as 
copper, were also present in the bromine solution, but apparently 
not to such an extent, for the solution was clear. 

The gold was finally precipitated by means of H 2 S. 

In the experiments on this particular ore bromine seemed to 
have the following advantages over chlorine: 

1. It extracted a much higher percentage than chlorine, the 
results being estimated not only on the assay of the tailings, but 
also on the actual gold recovered. 

2. It gave solutions much more free from base metals. This 
would be expected, especially where chlorine is generated by 
means of H 2 S 0 4 and bleaching-powder, and the acid has a chance 
to act directly on the ore. 

3. Less time is required to extract the gold. 

4. The ease in using and comfort in handling is much greater. 

As regards the comparative cost, the least amount of bromine 

which could be used on this ore with a successful extraction 
appeared to be 0.3 per cent, or 6 pounds per ton. With bro¬ 
mine at 25 to 40 cents per pound, this would make the cost very 
high; but the cost of chlorination would certainly be still higher, 
as it was found necessary to use as high as 10 per cent of lime 
and 6 per cent of H 2 S 0 4 to obtain even a fair extraction. 

Cyanogen Bromide. —This salt, discovered by Serullas in 1827, 
is supposed to have a greater solvent action on gold than cyanide 
alone. Serullas prepared it as follows: 

One part of bromine is poured upon two parts of cyanide of 
mercury contained in a tubulated retort or glass tube closed at 
the bottom and surrounded with ice; bromide of mercury and 
bromide of cyanogen are formed with great evolution of heat. 

The bromide of cyanogen sublimes in needles contaminated 
at first with bromine, but ultimately the bromine flows back and 


276 


NOTES ON ASSAYING. 


enters completely into combination. Gentle heat is then applied 
and CNBr sublimed into a receiver connected with the retort 
and surrounded with ice. 

Roscoe and Schorlemeyer say cyanogen bromide is formed by 
the action of bromine on hydrocyanic acid or on metallic cyanides. 

If bromine is added drop by drop to a well-cooled aqueous 
solution of potassium cyanide, crystals separate out which con¬ 
sist of a mixture of cyanogen bromide and potassium bromide. 
When these crystals are heated to a temperature of from 6o° to 
65°, cyanogen bromide sublimes in the form of delicate trans¬ 
parent prisms, which soon pass into the cubical form. 

The salt is poisonous and acts powerfully on the eyes. 

EXPERIMENTAL TREATMENT OF GOLD-BEARING ORES* 

The following questions often come up and require answer¬ 
ing in regard to samples from prospect-holes, as well as regards 
the ore from mines in actual operation. 

1. What value has the ore? 

2. Is it a free-milling ore? 

3. If so, what percentage of the gold can be extracted by 
amalgamation or by passing the ore over amalgamated or silver 
amalgamated copper plates? (All gold which is free will not 
necessarily amalgamate.) 

4. What value have the tailings after this treatment? 

5. What percentage of concentrates does the original ore carry? 

6. What value have these concentrates; that is, will it pay 
to put in some kind of concentrating machinery in order to save 
the concentrates, or can the tailings from the plates be treated 
directly with KCN ? 

These questions can of course be answered in the most satis¬ 
factory manner by having from 15 to 20 or more tons crushed 
and tested in some gold-mill, provided with all the modern 
appliances for crushing, amalgamating, and concentrating. 

They can, however, be very well answered by means of the 
following tests: 


* See Canadian Mining Review , October 31, 1898. 





METALLURGICAL LABORATORY EXPERIMENTS. 


277 


Test jor No . 1 .—Weigh the ore. Crush clown gradually and 
then sample very carefully. Sample should be weighed before 
passing it through each sieve, and especial care should be 
observed in regard to any residue left on any sieve or any 
pellets of gold found on said sieve. These should be carefully 
saved and assayed separately. The final sample for assay should 
be crushed through a 120-mesh sieve at least. (Assay notes, 
Sampling Ores.) Assay. If pellets have been found, calculate 
them in the final result. Give the ounces the ore runs per ton 
of 2000 lbs. and the value per ton. 

Tests for Nos. 2, 3, and 4 can best be made by treating the 
ore by one of the following methods : 

The ore should pass a 30-mesh sieve at least. 

a. In a miner’s ordinary gold-pan. Take 300 to 500 grammes 
of ore, which has previously been carefully sampled and assayed. 
Mix into a thick pulp with 35 to 60% of water (depending 
upon the character of the ore), and then add 5 to 10% of 
clean mercury. Shake up well for some time and pan down in 
the usual manner. Separate the mercury and amalgam from 
the ore and concentrates. Save all the water, concentrates, 
tailings, and slimes. Filter the whole, or else allow them to settle 
overnight, decant off water, dry, weigh, and assay the tailings. 
The sample for assay should be crushed through a 120-mesh sieve. 
Calculate from this assay the total gold in the total tailings, then 
calculate the total gold in ore taken for amalgamation. The dif¬ 
ference is the gold amalgamated. Figure the percentage. The 
mercury and amalgam may be retorted, or if small in amount, 
treated with dilute nitric acid in a parting-flask. 

In this parting heat the solution, but do not allow the action 
to become very violent. The gold, unless at the very end the 
amalgam is touched, will be left in beautiful, fine, yellow, needle¬ 
like crystals. Wash free from acid and the nitrates of mercury, 
transfer to an annealing-cup, heat in a muffle, and weigh* 

b. In a good stout bottle or fruit-jar. Take 200 grammes to 
one kilogramme of ore, place in jar, add 5 to 10% of mercury, and 
sufficient water to make the whole into a thick pulp. Stopper the 
jar tightly and shake up and down vertically for one half-hour 


278 


NOTES ON ASSAYING. 


or else revolve three hours. Pan down as usual and treat the 
tailings and mercury as described under method a. 

c. Crush the ore through 30- or 40-mesh screen and pass it 
over silver-amalgamated copper plates. Plates are then scraped 
and freed from the silver-gold amalgam, which is retorted. 
(See Retorting Mercury.) 

The tailings are collected, dried, weighed, and assayed. 

Sample for assay should be crushed through 120-mesh sieve. 

This method may not give quite as high an extraction as 
when the ore is stamped and then goes over the plates, because 
the grinding and polishing action of the stamps on the gold is 
lacking. 

d. In the Ball Mill. (See page 279.) 

Test jor No. 5.—Take from 500 to 2000 grammes of the ore, 
after amalgamation, through 40-mesh sieve (an ore through 30- or 
40-mesh sieve is sufficiently fine for all these tests; otherwise the 
concentrates will be slimed), and carefully pan or van it down, or 
else pass it through a hydraulic classifier. (See page 280.) 

Dry and weigh the heads, and be sure not to have the heat so 
great as to roast the sulphides and thus alter their weight. 

Test jor No. 6 . —Assay the concentrates obtained in No. 5 by 
crushing them all or a sample of them through a 120-mesh sieve. 

A true value of the tailings from the above tests can only be 
obtained by saving all the water and ore and slimes. Owing 
to the difficulty of obtaining check assays on an ore carrying free 
gold, the true value should be based on the total amount of gold 
recovered by amalgamation plus the total gold found in the con¬ 
centrates and tailings afterwards. 

Example oj Treatment. —Sample received weighed 20 kilo¬ 
grammes; it was crushed, sampled, and assayed to obtain the 
value in gold per ton. 

500 grammes of the ore were taken and the concentrates 
removed by panning, to determine the percentage per ton of 2000 
lbs.; also to find how many tons would concentrate into one. 

15 kilogrammes were amalgamated and the mercury and 
amalgam retorted. Both the concentrates and tailings were 
saved, weighed, and valued. 



METALLURGICAL LABORATORY EXPERIMENTS. 


279 


We then had: 


Hg and amalgam, Concentrates. Tailings. 

which was retorted. Weighed, sampled, Weighed, sampled, 

and assayed. and assayed. 

The report read as follows: 


The ore assayed 4 oz. gold per ton of 2000 lbs. @ $2o 67 / l00 per oz.= ,$82.68 

500 grammes ore, through 30-mesh, gave 20 grammes concentrates 
= 4 per cent in ton of ore. 

15 kilos, of ore were amalgamated; gold contents as per assay. 2.0565 gms. 


Concentrates, 600 grammes after amalga¬ 
mation (assay 14.58 oz. per ton). 

Tailings, 14350 grammes (assay .02 oz. per 


.7452 gms. 

gold 

84.86% 

.3000 ‘ ‘ 

i < 

= 14.58 

.00976 “ 

C c 

.47 

.00150 “ 

( i 

.07 

•.05646 “ 

a 

99.98% 


If ore contains 4 per cent of concentrates, 25 tons will concentrate into one, for 
80 : 1 :: 2000 : #. 


FREE-MILLING TEST IN BALL MILL. 

First clean out the mill thoroughly, which can be done with 
a stiff brush, some water and sand, to remove anything left from 



previous test. Be very sure to get no oil or grease inside of the 
mill, otherwise the mercury will “sicken” or “flour” badly. 

Weigh out 2 to 5 kilogrammes of ore for each side and charge 
it at opening (b) on the side, the plug ( a ) having previously been 
screwed in tight. Add 35 to 60% of water, to make the pulp 
into a thick mud, and then add 1 to 5 iron balls. If the ore is 
rich in sulphurets or arsenical compounds, use only one or two 




































280 


NOTES ON ASSAYING. 


balls. These will keep the ore well stirred up and will be less 
liable to make slimes and flour the mercury. Stop up opening ( b) 
and start mill in order to grind the ore. If ore is through 30- 
mesh sieve, amalgamate directly, for grinding is unnecessary, and 
1 or 2 balls will be sufficient. After the ore is sufficiently fine, 
amalgamate by one of the following methods: 

a. With 200 gm. of mercury alone. 

b. 11 “ “ “ “ and 10 gm. KCN. 

c. “ “ “ “ “ and 50 “ sodium amalgam.* 

d. “ “ “ “ ‘ ‘ and 50 11 mercuric chloride, 

and then later on add 10 “ KCN. 

Methods b and c simply clean off the oxides and other com¬ 
pounds soluble in these substances, and they keep the Hg bright 



and active. In d the corrosive sublimate (HgCl 2 ) brings about 
electric action between the gold particles and the iron, the iron 


* About 97% Hg and 3% Na. 













METALLURGICAL LABORATORY EXPERIMENTS. 


281 


being the poles and the HgCl 2 the electrolyte. Amalgamate in 
all the methods from J to 3 hours. 

To clean up test, take out cap ( b ), add some water and dis¬ 
charge contents through the opening ( a ) into iron kettles or 
wooden pails. Finally clean out inside of mill with a stiff brush. 
Save all water, sand, and slimes. 

The mercury and amalgam may be separated from the sand 
by means of a gold-pan, avanning-shovel,orby a hydraulic classifier. 

This last is the quickest, but not necessarily the most satisfactory. 

If the vanning-shovel is used, do not put too much material 
upon it at one time. 

Shake and settle the mercury very 
thoroughly upon the van before washing 
off the first lot of waste. Gradually 
bring forward the concentration until it 
consists largely of mercury and concen¬ 
trates. Then pour the Hg into a bowl 
and save the concentrates. 

Repeat the vanning upon another por¬ 
tion of the pulp, and so on until all is treated. Finally pan all 
the concentrates once more for any drops of Hg, and then clean 
the mercury for retorting. 

If the Hg from any of the tests is found to be “ foul ” or 
“ leady ” or in a “ floured ” condition, it is well not to separate 
it too cleanly from the pyrite and other concentrates, but to 
carry some of these along with it. Now cover the mercury and 
concentrates with a little water and try to clean and collect 
the mercury by adding either a small piece of KCN, a little ammo¬ 
nia, or some KOH. If these fail to clean and bring it together, 
wash thoroughly with water, leaving the mercury just moist, 
and add, one at a time , a few small slivers of metallic sodium, 
which will always bring all the mercury together. 

Save all the sand, slimes, and water. Filter them or allow 
them to settle overnight or until the water is clear, then decant or 
siphon ofj the water, dry residue, weigh; pass through sieve fine 
enough to remove all lumps, sample and assay. Grind the sample 
for assay through 120-mesh sieve. 









282 


NOTES ON ASSAYING. 


This method is better than taking a running sample from 
the classifier, because it is sure to save all the slimes, which are 
very often the richest portion of the tailings and which would 
otherwise be lost. 

The mercury and amalgam are cleaned and retorted and the 
residue treated as per “Retorting Mercury,” page 290. 

Report the following data: 

Character and composition of the ore as ascertained by inspec¬ 
tion and panning. 

Size of ore as received and treated. 

Method employed in amalgamation test and chemicals used, 
if any. 

Time taken in grinding the ore. 

Time taken in amalgamation. 

Condition of the mercury at the end of the test, i.e., whether 
it was bright and clean or dull and foul. 

If the ore contains both gold and silver, hand in a report 
upon each separately. 

#=weight of original ore. Assay. Total gold = a. 

y = tailings (after panning off Hg). Assay. Total gold = b. 
a-b = gold amalgamated = c. 

c 

— = percentage of gold amalgamated. 

CL 

Tailings (y) are panned or freed from concentrates. We 
then have concentrates (M) and tailings (N). 

Weigh and assay M. Weight of gold in M — d. 

Weigh and assay N. Weight of gold in N = e, 

The weight of y should equal M+N. y — M should equal iV. 

The gold in c, d, and e should equal that in a. We shall then 
have 

— percentage of gold amalgamated. 

— “ of gold saved in heads and concentrates. 

— “ of gold lost in final tailings. 

— “ of gold unaccounted for. 


100 

Mercury used = 200 grammes. 

“ recovered = 197 “ =98.5%. 



METALLURGICAL LABORATORY EXPERIMENTS. 


283 

For further data as to reporting results see page 2795 “Experi¬ 
mental Treatment of Gold-bearing Ores.” 


AMALGAMATION OF GOLD ORES. 


Stamp-mill Work.—The two following runs give an idea 
of the work which the small stamp-mill in this laboratory 
will do: 


The mill has three stamps weighing 225 lbs. each, made up 
as follows: 


Rod... 
Boss... 
Shoe.., 
Tappet 


35.3 kilos 
27.9 
22 

17.4 


(< 

u 

<< 


The dies weigh 10.4 kilos each. 

The shoes and the wooden wedges, with which they are fastened 
on to the boss, have the following dimensions: 



The tailings from the plates were concentrated on a full-size 


4-foot Frue vanner. 


ORE No. 1490. 

Two portions of a Nova Scotia gold ore 
divided while in a coarse condition. 
W to 3".) 

A B 


Assay. 1.25 oz. 

Total ore crushed, kilos. 343 * 

Rate per 24 hours (3 stamps), tons. 1.6 

Sieve (punched) corresponding to. 40-mesh 

Drop of stamps, inches. 5§ 

No. of drops per minute. 97 

Feed-water per 24 hours, gallons. 5 10 ^ 

Slope of plates, inches per foot. i| 

Concentrates in ore, per cent. 3*^7 

Concentrates, ounces per ton. .58 

Vanner tailings (314 kilos), ounces. .02 

Ore lost during process, per cent. 7.85 


Mercury was used in the battery in both runs. 


1.17 oz. 

339-8 
1.62 

30 (steel wire) 

si 

98 

i-37 

5-53 

.74 

•53 

7-7 




































284 


NOTES ON ASSAYING. 


GOLD ACCOUNT. 

Total gold in ore, based on assay, gms.. 14.69110 

Saved in battery, grammes.... 10.93520 


i i 

on 

< < 

plate, “ .... 

.33854 


i i 

l < 

plate 

1, copper, “ .... 

.08087 


(< 

i < 

< < 

2, Muntz metal,* gm... 

.03011 

<L> 

bJD 

C i 

c c 

c c 

3, copper, grammes.... 

•03385 

c 

a 

i i 

< c 

( i 

4 a a 

4, .... 

.02105 

CL 

( c 

i < 

i c 

- i C ( c 

.... 

.01050 

c n & 

r « j 

( c 

i < 

i i 

6, Muntz metal, gm.... 

.00468 

A , a 

CL " -1 

( c 

< c 

6 C 

7, copper, grammes.... 

.00611 

0 

r* 

i i 

< < 

( c 

8, “ 

.00878 

»—1 

• «—» 

£ 

i < 

< c 

6 ( 

9, “ “ .... 

.00736 . 

■ 


Mercury trap 
* 60% Cu, 40% Zn. 


.00470 


1 3 - 6 3 it 3 

12.10840 
.11872 
.06604 
.04287 
$ .04162 

4 -> 

£ .02629 
CL 

l , -01254 

g. .01666 
o .01038 
.01038 
.00682 
.03720 


11.48175 12.49792 

In tailings.5652 

In concentrates.4675 


13.53062 = 98.43% 

Length of plates, 75 inches. 

Area of outside amalgamating surface, 1587 sq. in. 

“ “ inside “ “ 83^ “ “ 

Silver amalgam was spread over the plates with a brush, and was scraped off 
after the run and retorted. 

Gold actually extracted, based on assay.. . 78.16% 91.68% 

Per cent of the gold saved, which was col- 

! 7374 \ 


lected in the battery 


/it 1 273 
\ii .481 


75 / 


98.18 


97-83 


SIZING OF THE BATTERY TAILINGS. 
Per Cent. 


On 40 sieve.898 


Thro. 40 on 

60 sieve 

3-93 

“ 60 “ 

80 

C i 

5.02 | 

“ 80 “ 

100 

< ( 

5 - 5 i ( 95-03 

Through 

100 

( i 

84.50 ) 




99.86 


Per Cent 


On 30 sieve. 
Thro. 30 on 



.00 ' 
.08 j 

L 

40 

sieve. 

[.Too little 

I to assay. 

40 “ 

50 

< < 

.38 

) 

50 “ 

60 

< < 

I * 5 I 

. 12 oz. 

“ 60 “ 

80 

< c 

3 - 3 i 

.06 oz. 

“ 80 “ 

100 

c < 

9.28 

.06 oz. 

Through 

100 

< c 

85-44 

. 14 oz. 


100.00 


Making Silver Amalgam.—Unless the amount of acid and 
mercury is in the right proportion to the silver taken, the amal¬ 
gam will not come out satisfactorily and a basic nitrate of mer¬ 
cury or a blue powder will be liable to form. 

Mr. C. I. Auer, class of 1901, took this subject (the making 
and composition of silver and gold amalgams) as a thesis. He 


























METALLURGICAL LABORATORY EXPERIMENTS. 285 

found that the following rules should be followed in making 
silver amalgams: 

Have the silver finely granulated. 

Use 4 c.c. of HN 0 3 ( S P- gr. i-20) for every gramme of 
silver. 

Cover the vessel in which the solution takes place, and heat 
gently, but do not boil, that the silver may go into solution quietly 
and the acid not evaporate. Filter off the gold, if any is present, 
add 8.6 c.c. of "water for every gramme of silver taken, and 
then add the mercury all at once. Use 16 grammes of mercury 
for every gramme of silver. 

The nitrate of silver solution, when the mercury is added, 
can either be hot or cold. 

After adding the mercury, stir the solution constantly until 
all the silver has amalgamated. If a silky precipitate comes 
down, or a blue powder tends to form and grow like a mush¬ 
room on the amalgam, it indicates that the solution is not suffi¬ 
ciently acid. This blue powder consists of Ag and Hg in vary¬ 
ing proportions. Decant the nitrate of mercury from the amal¬ 
gam into a wide-mouthed bottle and wash the amalgam once, 
by decantation, adding this washing to the first decantation. Give 
the amalgam six or more additional washings, stirring it thor¬ 
oughly with a large porcelain spatula. Save all these wash¬ 
ings in another bottle separate from the first two decantations. 
The amalgam is next squeezed through chamois or cotton cloth 
and the excess of mercury removed. 

Amalgam made as above may be strained or squeezed through 
chamois, linen, canvas, or cloth. After being squeezed through 
these by hand pressure, the residue in the chamois or other mate¬ 
rial will carry from 14.5 to 17.5 per cent of silver, depending 
upon the pressure, the material used, and the temperature of 
the amalgam at the time of squeezing. The mercury which 
passes through the material carries about .045 per cent of 
silver. 

When the amalgam, squeezed by hand, is put under a pres¬ 
sure of 48,000 lbs. per square inch, more mercury is removed 
and the residue contains from 23 to 24.5 per cent of silver. 


NOTES ON ASSAYING. 


2 86 


The mercury removed in this manner carries from .05 to 
.06 per cent of silver. The sp. gr. of the amalgam containing 23 
to 24.5 per cent of silver is 13.7 to 13-76. 

Gold amalgams, when squeezed by hand, carry from 32 to 
41 per cent of gold, and the mercury removed will carry from 
.12 to .16 per cent of gold. When placed under 48,000 lbs. 
pressure the percentage of gold increases to 44 or 48 per cent. 
This is not so high as some samples of scale removed from plates 
that have been given to me. 

One sample, taken from an outside plate of a stamp-mill, 
carried 39.39 per cent of gold on one side and 42 per cent on 
the other. 

A sample of very hard scale, near the head of outside plate 
(six months’ run), carried 56.87 and 57.75 per cent of gold. 

Recovery of Silver and Mercury from the Nitrate Solution.— 
Either of the following methods can be used: 

1. Put in iron rods or scrap. Both Ag and Hg will be thrown 
down, after some time. Clean the iron, filter solution, and retort 
the residue. 

The solution must not be too acid, because a great deal of 
iron oxide will be found in the residue. 

2. Throw down both the Ag and Hg as chlorides. Filter, 
wash, and dry them. 

Weigh and mix with one fourth their weight of oxide of lime 
(CaO), rubbing them together in a mortar. 

Retort this mixture. The mercury distils, leaving a residue, 
containing silver, which is fused with a little soda, silica, and a 
large amount of borax glass. Cool the crucible, break it and 
weigh the silver button. 


BULLION. 

Melting and Refining. —The following notes are from books, 
data collected at smelting works, and my own experience: 

A good furnace with a splendid natural draft or with air sup¬ 
plied by a blower is the first requisite. 

If large graphite crucibles, i.e.. Nos. 100 or 125 are to be used, 


METALLURGICAL LABORATORY EXPERIMENTS. 287 

the furnace should be about 3' 6" deep, 2' 6" wide at bottom 
(inside), and 2' 2" at the top. 

The flue should be about 10" below top of furnace and 8" 
by 12". 

The black-lead crucibles, when new, should always be heated 
slowly while upside down; when red they should be turned and 
placed right side up. They should stand upon a 3" fire-brick 
placed across the grate-bars. The tongs with which to handle 
these crucibles should always fit well about them, and the graspers 



(a), in figure of crucible, should come well below the bulge or 
largest part of the crucible. When the tongs are in place, slip a 
ring over the handles to hold them firm. 

Always put a handful of borax glass into crucible before 
charging the bullion. If the precious metals are in a fine con¬ 
dition, charge with a scoop or a funnel. (See cuts.) 

Gold, precipitated upon zinc in the KCN process or from an 
AuC 1 3 solution, is generally mixed thoroughly with from one and 
one-half to twice its weight of borax glass by revolving them to¬ 
gether in a barrel with iron balls. The mixture is then charged 
directly into the crucible. Cover crucible and heat until con¬ 
tents are thoroughly liquid. 

If bullion is base, nitre and borax glass are both needed in 
refining, but too much nitre will rapidly eat into the graphite 
crucible. Lead, when present in the bullion, is best oxidized by 
nitre or sal-ammoniac; tin by means of K 2 C 0 3 ; Sb and As by 
means of nitre or by stirring the bullion with an iron rod. 
Iron rust can be removed by adding CaSCL; the sulphur taken 
up by the metal is then removed by stirring the bullion with an 
iron rod. Na2C0 3 is to be avoided unless there is silicious mat¬ 
ter present; still a little of it with nitre seems to work well even 
if bullion is quite pure. 







288 


NOTES ON ASSAYING. 


Bone-ash and silica save the crucible from the action of the 
oxides, and are especially useful for thickening the slag in case 
skimming is necessary. The skimming is done by means 
of an iron rod coiled as in the adjoining figure, and the 
spiral then bent so that it will be parallel with the surface 
of the bullion to be skimmed. If bullion is poured to¬ 
gether with the slag—and many say this is the only way 
to obtain a clean brick—the slag should be perfectly 
liquid. 

Toughening.—This process serves to eliminate small quantities 
of impurities like As, Pb, Sb, etc., which would render the bullion 
brittle and unfit for coinage purposes. 

T. K. Rose and others recommend the addition of a little 
sal-ammoniac or corrosive sublimate to the melted bullion. 

Cover quickly to keep volatile chloride fumes out of the 
room. Test by dipping out a small sample and casting it into 
a thin ingot. Cool it in water and see if it will bend upon 
itself. 

Pure gold is a brilliant green color when it is melted, and it 
may then be poured. 

Silver, when nearly pure, often bubbles violently in the crucible, 
and some say this is especially so when much nitre has been 
used in refining. The remedy seems to be to lower the tempera¬ 
ture, cover it with charcoal and stir it with a graphite rod until 
the bubbling ceases. Then pour. 

Pouring and Casting.—Always stir the bullion thoroughly 
before doing this, and if a sample is to be taken for assay, take 
it immediately after the stirring. Ingot moulds should be per¬ 
fectly bright and clean and heated on the top of the furnace 
until they are too hot to be handled with the bare hands. Some 
heat them almost to the ignition-point of oil. 

As regards the use of oil, while some put in almost £" in the 
large moulds, others are accustomed to use only a little around 
the top of the mould and none at the bottom. Fine rosin, sprinkled 
into the bottom of the mould just before pouring, is also used. 

The object of the oil is to make a smoother ingot and bv its 
burning on top of the ingot to stop all sprouting and tarnish- 



METALLURGICAL LABORATORY EXPERIMENTS. 289 

In the case of small ingots fine charcoal sprinkled on top, as 
soon as the pour is made, will also answer nicely. Pour the 
metal quickly and carefully, always moving the crucible back and 
forth over the mould to avoid pouring in one spot. Pouring in one 
place makes a poor ingot and one that is liable to stick in the mould. 

Ingots may be taken out when they are still hot, and if not 
quite clean they may be plunged into dilute H 2 S 0 4 . 

Too much care and attention cannot be given to the saving 
of all slags, droppings, and skimmings. The floor or bench 
on which the work is done should have been made with this 
especial end in view, and should be perfectly smooth and tight, 
that the slags, etc., may be swept up and saved. Wood should 
be avoided in the making. 

All crucibles, tools, and anything else connected with the 
work are also saved, broken up, and worked up afterwards. If 
these details are looked after carefully, the loss in melting should 
be small. 

If the bullion is to be granulated, it is best done by pouring 
it into a copper vessel or tank filled with ice-cold water. Pour 
the metal in with a wavy motion, holding the crucible 3 or 4 
feet above the surface of the water. 

Small Amounts of Bullion.—Pouring small quantities of bul¬ 
lion, whether it is gold alone or an alloy of gold and silver, is 
generally unsatisfactory owing to the difficulty of making a clean 
pour. 

Very small lots I prefer to melt in a clay crucible, which should 
be well glazed upon the inside with borax glass before the bullion 
is added. Cover with a good layer of borax glass and a little soda, 
and keep the crucible covered until its contents are perfectly quiet. 

As an extra precaution the small crucible can be heated within 
a larger one. 

Take the crucible from the furnace, see that no small globules 
are on the sides, and then allow it to stand until it is cold. Break 
the crucible and save it, together with the slag, if it is not per¬ 
fectly free from metal. Clean the ingot and weigh. If the slag 
is not clean, pulverize, fuse it with litharge and argols, and cupel 
the resulting lead button. 


290 


NOTES ON ASSAYING. 


RETORTING AND CLEANING MERCURY. 

The retorts may be large or small and of different shapes. 
When large, and a large amount of mercury is to be distilled, there 
is always a tube 0 , as in retort x, passing through the cover, by 
means of which the mercury may be charged into the retort. 
The tube may be straight or have a bend which serves as a trap. 



Form y is used when small amounts are to be distilled. 


d 



The retort a should be first thoroughly cleaned and coated 
on the inside with chalk, ruddle (Fe 2 0 3 ), graphite, or, better than 
any of these, lined with a piece of paper put in the bottom and 
part way up the sides. This will prevent the residue sticking 
to the bottom of the retort, which it is very apt to do if the heat 
becomes too great or if lead or zinc is present. 

Next see that the delivery-tube d , attached firmly to the 
-cover, is perfectly free and open. The turned parts of the retort 
























METALLURGICAL LABORATORY EXPERIMENTS. 


291 


and cover are next cleaned, and the Hg and amalgam put into 
a. Mix some mineral paint or ruddle to the consistency of 
thick cream and smear the rim of the retort a and rim of cover 
b with an even coating of it; then place cover on retort, put 
on clamp c, and screw down firmly. At the Utica Mine, Cali¬ 
fornia, they use wood ashes (through 30) mixed with water for 
a lute. 

The retort is now ready to be heated. If large, it is generally 
heated over a forge or in a crucible-furnace; if small, a good 
lamp will do it. In either case the bottom should never be 
heated above a dull red, otherwise it may be softened, bulged, or 
melted. 

No fluxing material, like borax, should ever be used in the 
retort , for the spheroidal Hg will be changed to cohesive, and 
boil with great violence. The residue might also be found per¬ 
manently brazed upon the inside of the retort. 

Mercury boils at 674° F., or 357 0 C. 

After lighting the lamp or fire, watch for the first mercury. 
First comes the tremble of the retort due to the boiling off of 
the moisture. Next comes the tremble of the retort due to the 
boiling of the Hg, followed by the hissing of the water when the 
hot Hg inside the tube comes in contact with the cold water 
outside the tube. 

The accidents most likely to occur are: 

1. The choking up of the delivery-tube d. 

2. The blowing out of the Hg between the retort and the cover. 

3. Burning out of the bottom of the retort. 

4. Adhering of the residue to the bottom of the retort. 

To avoid No. 1, see that the Hg or amalgam is thoroughly 
cleaned of all dirt and sand before it is put into the retort. 

Rapping the delivery-tube d with a light hammer every now 
and then helps to keep it open and clear. Have the pipe d well 
rounded at bend and full size all the way, and give it as much 
slope as possible. 

To avoid No. 2, which is due to tube d choking up or to 
poor luting, see that the luting on of the cover b is properly done 
at the start. If the retort leaks, a gray vapor will be seen rising 


292 


NOTES ON ASSAYING. 


above it. Test by putting a piece of cold iron in the blow for an 
instant; if it is coated white (Hg) there is a leak. In such a case 
instantly check fire or put out lamp, cool down as rapidly as 
possible, and avoid breathing the fumes or getting them into the 
eyes. Admit plenty of fresh air into the room or building. 

To avoid No. 3, see that the fire does not become too hot. 

To avoid No. 4, see that the retort is properly coated on the 
inside. 

Graphite is an excellent substance for luting and coating in 
all respects save one, and that is, that the residue in the retort is 
difficult to melt together, unless nitre is used. 

Chalk and mineral paint do not give this trouble, because 
the borax glass used in melting the residue in the crucible cleans 
and joins the metallic particles. Paper is, however, the best jor 
the purpose. 

The subsequent melting of the residue is done in graphite or 
clay crucibles as described under Bullion. 

Any small residues should be fused slowly in a crucible, to 
allow any mercury to go off gradually. If this residue is Ag or 
Au or both, use soda and borax glass as fluxes. If it is impure, 
use in addition litharge and argols and scorify or cupel the 
resulting lead button in this case. 

Scorification is dangerous, owing to danger of spitting, if any 
mercury is left in the residue. If attempted, the heating shoul 1 
be very gradual and another scorifier used as a cover. 

Melting lead in the retort to collect residue, as recommended 
in some books, I have never found a success. 

To clean the mercury and put it in good condition, first wash 
with a stream of H 2 0 to remove all soluble and light material, 
stir with porcelain spatula, and pour frequently from one vessel 
into another. Do not touch with the hands. Decant off water 
and add a small piece of potassium cyanide {poison), which ought 
to clean it nicely. Wash again with water, when the Hg should 
be perfectly clean. Most of the water is then removed with a 
sponge and the last of it by means of blotting-paper. Dry and 
weigh. Vessels for holding it should be strong, solid, and perfectly 
free from all oil or grease. 


METALLURGICAL LABORATORY EXPERIMENTS. 


2 93 


Large amounts of mercury, having been cleaned with KCN 
and washed with water, can be further purified in the following 
way: 

Pour the entire quantity through a funnel, either perforated 
itself or else with a chamois tied over the end of the stem, and 
allow it to fall as a spray through some 
acid as in b. From there it runs off by 
tube e into the iron mercury flask a. It is 
then both clean and almost dry, for its 
weight forces any acid or water out. 

If the Hg has been distilled from tin or 
zinc, the acid used is one part HC 1 and one 
part water. 

If distilled from lead, use one part HNO a 
and four parts water in tube b . 

Small quantities may be strained through 
chamois into a vessel containing the acid; it 
is then washed and dried. Hg containing 
small amounts of Pb or Zn distils more 
slowly than when these are absent, and it is 
difficult to free the Hg entirely from them. 

Blowing air into Hg also cleans it. One 
writer speaks of covering the Hg in the retort with cinnabar, iron 
filings, or lime, according to the impurities present. The sulphur 
in the cinnabar combines with the base metals, iron combines 
with As, forming a speiss, and the lime will take out the sul¬ 
phur. A layer of charcoal in the retort is also said to purify the 
mercury. 



MUFFLE CHLORIDIZING ROAST OF SILVER ORES. 

This process is applicable to ores which are not “free mill¬ 
ing,” i.e., ores which cannot be directly amalgamated. The ob¬ 
ject of the test is to determine, from a given quantity of the ore 
to be experimented with— 

i st. What amount of silver is lost or volatilized during the 
roast. (If the ore contains gold see A. I. M. E., Vol. XVII, as 
to loss in roasting.) 










NOTES ON ASSAYING. 


2 94 

2d. Percentage of soluble salts in roasted ore. 

3d. Percentage of silver as sulphate in roasted ore. 

4th. Percentage of silver salts soluble in hyposulphite of soda 
solution. 

5 th. Percentage of silver salts soluble in extra solution. 

The ores met with may be divided into three groups: 

(a) Heavily sulphuretted ores, which in some cases have to 
be roasted a long time previous to the addition of salt. 

(b) Slightly sulphuretted ores. 

(c) Ores carrying a very small quantity of sulphides, the 
sulphurets being present in so small an amount that, in some 
cases, it is found necessary to add sulphur or iron pyrites in order 
to decompose the salt and liberate chlorine. The percentage of 
salt added varies. (At Aspen, Colo., it is claimed that by using 
10 to 15% the amount of silver volatilized is diminished.) With 
some ores it is found better to add the salt at the beginning, 
with others at the end of the roast. Ores containing no As, Sb, 
Pb, or Ca are generally best treated by adding the salt at the 
beginning. If A s volatilizes as a sulphide, the loss of silver seems 
to be smaller than if it volatilizes as a chloride. Lead and lime 
should, if possible, be kept as sulphates, for otherwise they are 
great consumers of chlorine. 

Suppose an ore carries sulphides of lead, zinc, and iron. It 
is roasted for a long time, at a very low heat, in order to form 
sulphates of lead, zinc, and iron. The last will decompose salt 
with the formation of chlorine, the other two sulphates will not. 
Therefore we wish to form FeS 0 4 and as much sulphate and 
oxide of lead and zinc as possible and yet not decompose the 
FeS 0 4 . For this reason we must keep the heat low and roast 
slowly, otherwise the FeS 0 4 will be broken up before all of the 
PbS and ZnS are converted either into sulphates or oxides. When 
we think this point has been reached the salt can be added. 

The sulphates are decomposed by heat in the following 

« 

order: sulphate of iron, sulphate of copper, sulphate of silver, the 
last commencing to decompose at a red heat. Sulphate of zinc 
is decomposed with difficulty; sulphates of lead and lime are not 
decomposed by heat unless much silica is present. The first three 
sulphates will all decompose salt and are therefore chloridizers: 


METALLURGICAL LABORATORY EXPERIMENTS. 


2 95 


2NaCl+ FeS 0 4 = Na 2 S 0 4 +FeCl 2 ; 

2NaCl + CuS 0 4 = Na 2 S 0 4 + CuCl 2 ; 

2NaCl+ Ag 2 S 0 4 = Na 2 S 0 4 + 2 AgCl; 

4NaCl+ 2 FeS0 4 +30 = 4Cl+2Na 2 S0 4 +Fe 2 0 3 . 

These, together with the HC 1 formed, chloridize the ore: 

Steam + CuCl 2 = CuO + 2HCI; 

2HCI+Ag 2 S = 2 AgCl+ H 2 S. 

Students should make careful note of the duration of roast,, 
heat used, and at what time the salt was added. The following 
reactions may also take place, among a great many others: 

S 0 2 + O = S 0 3 . S 0 3 + 2 NaCl+H 2 0 = Na 2 S 0 4 + 2HCI; 

Fe 2 Cl 6 +30 = Fe 2 0 3 + 6 Cl. Heat upon 2CuC1 2 = Cu 2 C1 2 +2C1; 

2FeS0 4 +4NaCl+20 + H 2 0 = 2HCl+2Cl+Fe 2 0 3 +2Na 2 S0 4 ; 

Cu 2 S T FeS 2 T 6NaClT 12O = FeCl 2 + 2CuCl 2 -f- 3Na 2 S0 4 . 

Testing an Ore.—Crush ore through a 30- or 40-mesh sieve. 
Sample carefully and crush sample for assay through a 100-mesh 
sieve. Take 5 grammes and assay {all assays in this work should 
be made by crucible). Calculate both the per cent of silver in 
the ore and the ounces per ton. 

Weigh out from 2 A.T. to 200 grammes of ore (through a 30- or 
40-mesh sieve) on pulp-balance and roast in a clay dish with from 
3 to 10% of salt. Have the furnace only one third full of coke 
and the bottom of muffle just red at first , to prevent caking. Stir 
the ore every now and then and roast it from 30 minutes to 2 hours 
at a low heat. Never have the muffle near a scorifying tempera¬ 
ture, but the ore just red. After roasting, sift it through the same 
sieve as before and weigh on the pulp-balance. Sample care¬ 
fully, take enough ore for the following tests, and grind in an 
agate or porcelain mortar through a 100-mesh sieve. (It will take 
about 25 minutes to grind 80 grammes through a 100-mesh 
sieve.) If iron was used, we might have (2AgCl-f Fe = FeCl 2 -f 
2Ag), and the latter is not readily soluble in hyposulphite. 

Take 5 grammes and assay. Calculate the per cent of Ag and 
find total amount in roasted ore. Difference between this and 
total Ag in raw ore = silver volatilized during the roast. 


296 


NOTES ON ASSAYING. 


Soluble Salts (i.e., the excess of NaCl used and all chlorides 
and sulphates soluble in H 2 0 and the AgCl soluble in the NaCl 
solution if an excess was used in the roast).—Weigh 5 grammes 
and leach by decantation with hot H 2 0 , until neither AgN 0 3 
nor ammonium sulphide give a precipitate. Dry, burn the filter, 
and weigh. 

Loss = soluble salts. 

As the percentage of NaCl used in the roast increases, the 
soluble salts generally increase. 

Silver as Sulphate and Silver Salts Soluble in Water containing 
Salt (NaCl).—Assay the whole residue after leaching with water. 
The difference between this weight and the weight of the silver 
button in 5 grammes of roasted ore equals the Ag 2 S 0 4 and other 
silver salts soluble in water or in a brine solution, for it must be 
remembered that if a large excess of NaCl is used in the roast, we 
will have a strong brine solution, in which AgCl is soluble to a 
certain extent. If any NaCl is left undecomposed in the ore, the 
Ag 2 S 0 4 cannot be determined, for it will be broken up by the 
excess of NaCl, and the AgCl precipitated. This may, however, 
go into solution again on standing if the brine is sufficiently strong. 

Silver Salts Soluble in Hyposulphite.—Place J A.T. of ore in a 
beaker; add 300 c.c. of hot water, decant H 2 0, and then treat 
with about 250 c.c. of a 5% solution of hypo, (allow hypo, to stand 
on ore for say \ hour at about 125 0 F. and stir it frequently): 

2AgCl+ 2Na 2 S 2 0 3 = 2NaCl+ 2NaAgS 2 0 3 . 

Filter by decantation, wash several times with water, and finally 
wash with a little fresh hypo. The soluble silver salts should 
now be all removed and the last washing show no turbidity on 
the addition of ammonium sulphide unless lead salts are present. 
(PbCl 2 is not readily soluble in hypo., but PbS 0 4 is; so if much 
of the former salt is present, a precipitate of lead sulphide may 
be obtained after many washings.) Dry residue, weigh , and 
assay the whole of it. The difference between this and the assay 
of the roasted ore gives the amount of silver soluble in water and 
hyposulphite. Subtract the silver salts soluble in water and we 
obtain the AgCl and the silver salts soluble in hypo. 


METALLURGICAL LABORATORY EXPERIMENTS. 


297 

Silver Salts Soluble in Extra Solution.—This is the Russell 

process and consists in the treatment of the roasted ore with 
cuprous hyposulphite solution: 2\ parts Na 2 S 2 0 3 +5H 2 0 and 1 
part CuS 0 4 +5H 2 0. When made in this way, no cuprous hypo¬ 
sulphite will come down. The solution should be fresh and 
should contain a slight amount of free H 2 S 0 4 . If heated above 
85° C. it decomposes, and at boiling, Cu 2 S separates out. Silver, 
Ag 2 S, and the antimonial and arsenical minerals, like ruby silver, 
and stephanite, are soluble in this solution, whereas all these, 
with the exception of the silver, are insoluble in the hyposulphite 
alone: 

4Na 2 S 2 0 3 + 3Cu 2 S 2 0 3 -f 3Ag 2 S = 3Cu 2 S + 6 NaAgS 2 0 3 + Na 2 S 2 0 3 . 

The manner in which ores are treated with the extra solution 
of course may vary with the ore (see Lixiviation of Silver Ores 
with Hyposulphite Solutions by Stetefeldt), but the following pro¬ 
cedure will generally answer: 

Take i A.T. of ore and treat it with hot water as before. 
Decant off the solution and add 60 c.c. of H 2 0 and 12.5 grammes 
of hyposulphite of soda. Let it stand 30 minutes to one hour. 
Add 25 c.c. of copper solution (200 grammes CuS 0 4 in 1000 c.c. 
of water) and dilute with cold water to 300 c.c. and heat not 
above no° F. for about 10 minutes. Filter, wash with H 2 0 , 
dry, weigh, and assay. The difference between the weight of 
silver obtained from this and that obtained from the hypo, test 
gives the silver salts soluble in the extra solution and those not 
soluble in hypo, alone. 

If the ore, as received, has been previously roasted, it may be 
damp, owing to its having taken on moisture, due to the NaCl and 
other chlorides. In this case dry it, sample carefully, take 200 
to 250 grammes and proceed, as given above, with the determina¬ 
tion of “Soluble Salts.” 

Each student should make out a complete report similar to the 
one on page 298. 


REPORT UPON MUFFLE CHLORIDIZING ROAST. 

Ore No. 285. Assay, 105^ ounces. 

Character of ore= quartz carrying pyrite (FeS 2 ) and a little galena (PbS). 


298 


Per Cent 

Soluble in 

Extra 

Solution. 

NOTES ON ASSAYING . 

3 

n 

10 

• 

10 

co 

Per Cent 
Lost in 
Tailings. 


' * 

3 

'0 

co 

M 

H 

CN 

Qv 

vO 

Per Cent 
Soluble in 
Hypo., 
probably 
as AgCl. 

vO 

• 

0 

00 


Per Cent 
Soluble in 
H2O and a 
solution of 
NaCl. 
(Ag 2 SQ 4 ). 

S' 

co 

CO 

• 

CN 



Percentage 
of Ag 
Lost in 
Roast. 

S' 

00 

H * 

IT) 



Total 
Ag in 
Whole 
Ore, 
Grms. 

H VO M 
<0 ^0^0 
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Weight 
of Ore 
Used, 
Grms. 

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Make all assays by crucible method. 

Give: Length of time for which the ore was roasted. 

At what time the salt was added. 

Heat used during the roast, i.e., whether high or low. 
Any other data connected with the test. 









































METALLURGICAL LABORATORY EXPERIMENTS. 299 

The results a, b , c , and d, also a , 6, e, and / should foot up 
100 per cent, and are all based upon the total silver in the raw 
ore or (x). 

The amount of silver in the ore after leaching with water, 
that is .6689 grammes, can be obtained by either of the proportions: 
5 : 226 :: .0148 : x or 4.56 : 206.12 :: .0148 : x. 

The amount of silver in the ore after leaching with hypo, and 
the extra solution can be obtained in the same way. 

The tailings, after leaching with the extra solution, assay 7.7 
oz. per ton. The salts soluble in the extra solution = 16.05%. 
If the original ore was very rich in silver the percentage of silver 
salts removed by the process would have to be taken into con¬ 
sideration, but as the per cent of silver in this ore was only .4%, 
the silver salts soluble in this extra solution may be disregarded 
in the present calculation. Therefore if the tailings which assay 
7.7 oz. contained this 16% of other salts, they would assay lower, 
and to obtain this value we make the proportion of 

100 — 16.05 : 100 :: x : 7.7; 

that is, # = 6.5 oz. 

The application of this calculation is brought out more clearly 
in the next test, Pan Amalgamation. 

Going Back or Decrease in the Chlorination.—Some roasted 
ores when leached directly with hypo, show a higher extraction 
or chloridization than if leached with H 2 0 and then with hypo. 
This is said to be accounted for as follows (see paper by W. S. 
Morse, A. I. M. E., Oct. 1895): AgCl is soluble in brine, i.e., in 
H 2 0 used, plus the excess of NaCl left in the roasted ore. If the 
ore contains unroasted sulphides, say ZnS, not decomposed in the 
roast, we may have 2AgCl+ZnS = Ag 2 S + ZnCl 2 or 2AgCl + PbS = 
Ag 2 S + PbCl 2 . That is, the silver in solution has gone back to 
Ag 2 S, which is not soluble in hypo, alone, but is soluble in the 
extra solution. 

Example.—O re containing 29 oz. silver and about 2 .8% zinc. 
Ore leached with hypo, directly showed 78.92% of silver soluble. 
Ore leached with H 2 0 and then with 

hypo, showed 64.32% “ “ “ 


Silver gone back to Ag 2 S = 14.6% 



3 °° 


NOTES ON ASSAYING . 


This same ore treated with the extra solution showed 89% of 
the silver soluble. 

PAN AMALGAMATION OF SILVER ORES. 

Silver ores, for amalgamation, are divided into two classes: 

1. Free-milling ores, or those that require no roasting, such 
as chloride, bromide, iodide, and sulphide. 

2. Refractory ores, or those that require a chloridizing roast 
with salt previous to amalgamation. 

Whether the ore is raw or roasted, it should be sampled care¬ 
fully, after grinding it through a 30- or 40-mesh sieve. If the 
ore contains AgCl, avoid the use of iron for grinding, if possible, 
for 2AgCl-f Fe = FeCl 2 +2Ag. Assay by crucible method and 
calculate both the per cent of silver and of the gold in the ore 
and the ounces per ton. Determine the per cent of soluble salts 
and the per cent chloridized as per notes on “Muffle Chloridizing 
Roast.” Weigh out 2 kilos of ore, if small pans are to be used. 
(Large pans are iron; small pans are either iron or copper.) 
The pan is next cleaned out and then set in motion; add suffi¬ 
cient water to cover the mliller, the latter being so set as to 
grind upon the dies, unless a copper pan is used. The ore is 
then added until the pan is charged (the small pans take about 
1800 grammes). The contents should be about the consistency 
of thick paint, and they will probably require the addition of 
more water to make them so, but it is better to have the pulp 
too thick than too thin. 

Unless a copper pan is used or the ore has been roasted, the 
miiller should grind on the dies until the ore is entirely free from 
lumps. If all the ore will pass a 40-mesh sieve, grinding is unneces¬ 
sary. When the pulp is sufficiently fine, raise the miiller (to avoid 
grinding or “flouring” the Hg) and then add 200 to 500 grammes 
of Hg in a fine spray. This can be done through chamois or by 
means of a funnel drawn to a fine point. 

If the pulp is in the right condition, mercury will be found 
in every portion of it. Heat the pulp to about 160° to 180° Fah. 
and keep this temperature up to the end of the test. Amalgamate 




METALLURGICAL LABORATORY EXPERIMENTS. 


3 QI 


for from one to three hours and try to keep the pulp of the same 
consistency all the time. Many reactions take place which may 
be due to the constituents of the ore or may arise from the chemi¬ 
cals, such as salt, blue vitriol, or H 2 S 0 4 , which have been added 
to the pulp.* Much uncertainty exists in regard to these reac¬ 
tions, but the following are said to take place: 

CuS 0 4 + 2 NaCl = CuCl 2 + Na 2 S 0 4 ; 

Cu 2 Cl 2 + Ag 2 S = 2 AgCl + Cu 2 S; 

2 CuC1 2 + Ag 2 S = 2 AgCl + Cu 2 Cl 2 + S; 

2 CuC1 2 + 2Ag = 2AgCl+Cu 2 Cl 2 ; 

Cu 2 Cl 2 + Ag 2 S = 2 Ag + CuS + CuCl 2 ; 

Hg 2 Cl 2 +Fe = FeCl 2 + 2Hg; 

2AgCl +Fe = FeCl 2 + 2Ag; 

Ag 2 S + 2Hg = Ag 2 Hg + HgS. 

The iron pan aids the action of the mercury, and decom¬ 
poses both the AgCl and any calomel that may form. 

PbCl 2 will amalgamate, but PbS 0 4 will not, and this is one 
reason for adding CuS 0 4 when lead is present in an ore. When 
zinc blende and pyrite are present and the gangue is calcareous, 
some authorities say that salt and bluestone should not be added 
to the pan. The lime consumes the bluestone, forming CaS 0 4 , 
and the sulphides seem to flour the mercury in the presence of 
salt and perhaps form chlorides. CuCl 2 or Fe 2 Cl 6 , if present in 
too large proportion, are liable to form calomel (Hg 2 Cl 2 ). To 
discharge the small pan, either fill it with water and run the 
whole contents into a hydraulic classifier or else pan it down in 
a gold-pan. All the water, slimes, and tailings are either filtered 
on a large cloth filter, stretched on a wooden frame, or allowed 
to settle overnight and the water decanted the next day. Dry 
the residue, weigh , pass through a sieve to remove the lumps , 
sample carefully, and assay. The mercury and amalgam are 
cleaned and retorted. (See Retorting.) As some amalgam will 
stick to the pan and muller, or some amalgam, from a previous 


* Patio Process for Amalgamation of Silver Ores. Manuel V. Ortega, 
American Institute Mining Engineers, November 1901. 



3 02 


NOTES ON ASSAYING. 


run, may be removed, results must be based on the assay of the 
tailings. 

The residue in the retort is melted in a small crucible with 
a little soda, borax glass, and lead, and the button cupelled, unless 
copper is present, when the button will have to be scorified until 
the copper is removed. 

Weigh the resulting button and, if gold is present, part as 
usual. 

The report should be as follows: 





Total 

Per 

Cent 

Chlorid- 

ized. 


Per 


Weight 
of Ore. 

Assay. 

Weight 
of Ag 
in 

Per 

Cent as 
Ag 2 S0 4 

Cent 

In 

Tail- 




Grms. 


ings. 

Raw ore. 







Roasted ore. 

1710 gm. 

106.4 oz. 

6.240 




Tests on the roasted ore showed 




that: 







Salts soluble in H 2 0 = 8.9i% 
Sample of roasted ore lixiviated 
with hypo, showed 

Soluble salts = 9.2% 

Based on these results, the 
whole roasted ore, after 
hypo, leach, would be. 


19.8 oz. 


79 . I 

3-98 

16.92 


1553 gm. 

19.8 oz. 

I.054 





Silver extracted. 5.186 =83.11% 16.89 

Tailings, calculated from salts 

soluble in water = 1558 gm. 

Actual tailings, after amalga- 

mation= 1523.4“ 23.2 oz. 1.21 . 19.39 

Bullion recovered. 4-42 = 7°.83% 

Unaccounted for.61 = 9-78% 

Weight Taken. Recovered. Per cent Lost. 

Mercury. 500 grammes 490 grammes 2 


In practice it is not possible to weigh the tailings from a proc¬ 
ess, but it is possible to weigh both the raw and roasted ore and 
to obtain the assay of these. It is also possible to obtain a sample 
of the tailings as they go to waste, and from these results calcu¬ 
late the true extraction. 

If the process is pan amalgamation, the assay of the roasted 
ore is obtained and the percentage of salts soluble in water. If 
a leaching process is used, the roasted ore is assayed and the 
salts soluble in the solvent used are determined. A fair sample 































METALLURGICAL LABORATORY EXPERIMENTS. 


3°3 


of the tailings as they are discharged from the pan, settler, or 
leaching-tank is taken and assayed. 

Now this assay must be higher than it would be if the salts, 
other than silver salts (the percentage of these may be neglected 
unless the ore is extremely rich) soluble in water, were still present 
in the tailings or waste; in other words, the tailings have been 
partly concentrated. 

Take the previous test for example; the tailings after amalga¬ 
mation assay 23.2 oz. and the salts soluble in water = 8.91%. 

Therefore, 100 — 8.91 : 100 :: x : 23.2; that is, # = 21.13 oz. 

If the roasted ore assayed 106.4 oz., then the bullion recovered 
should be 


106.4 — 21.13 
106.4 


= 80.1%. 


The actual bullion recovered, based on the percentage of 
silver left in the tailings (19.39%), was 80.61%, which corresponds 
very closely 
































































































































































. 








. 




. 












INDEX. 


Acid slag, 8, 9 
Active fluxes, 89 
Alkaline carbonates, 7 
Alkali or alkali wash, 268 
Alloys of silver and copper, cupella- 
tion of, 204 
Amalgam, gold, 286 
silver, 285 
making, 284 

Amalgamation in ball mill, 279 
by bottle, 277 
of gold ores, 283 
by pan, 277 
by stamp-mill, 283 
Annealing-cups, 154 
Antidote for KCy poisoning, 269 
Antimonial ores for silver, 45, 122 
Antimony, reactions in cupellation, 

59 

Argols, 6, 71, 83, 85 

fusion for reducing power, 83, 
84 

Arseniate of soda and iron, 70 
Arsenite of soda and iron, 71 
Arsenical ores, crucible assay for gold 

and silver, 136 
scorification foj silver, 

. , 45 

Assaying, definition, 1 
Assaying solutions, 183 
Assay of ores for copper, 213 

gold, 127 
lead, 190 
platinum, 224 
silver, 39, 86 
tin, 219 

Assay reagents, 5 
Assay ton system, 3 

Balances, 2 

Ball mill amalgamation, 279 
Barite, flux for, 73, 124 


Barite ores, assay of, 124 
Barrel chlorination, 252 
Base bars, 198 
Base bullion, 199 
Basic slag, 8, 9 
Bicarbonate of soda, 66 

influence on re¬ 
ducing power, 
8 4 > 97 

Bismuth, 47 
Black flux, 194 

substitute, 194 
Bleaching-powder, 2s c 
Blicking, 56, 57 
Bone ash, analysis, 20 

on the market, 20 
Borax, 68, 85 

influence on R.P , 86, 96 
Borax glass, 68, 85 

influence on R.P., 86 
in scorification, 44 
Bottle amalgamation, 277 
Broad spatula, 30 
Brightening of button, 56, 57 
Brittle buttons, 42, 47, 91, 197 
Bromine, treatment of gold ore by, 
272 

Bromide of cyanogen, 275 
Bucking-board, cleaning of, 31, 32 
Bullion, 198 

assay of, 199 
base, 199 
gold, 210 

assay of, 211 
melting and refining, 286 
pouring and casting, 288 
silver, 198, 201, 205 
assay of, 201 
Gay-Lussac method, 
199 

results, 203 
Volhard’s method, 207 
305 





306 INDEX . 


Bullion, silver, wet methods, 207 
small amounts, 289 
toughening, 288 


Calcining, 244 
Carat, 5 

Charcoal, 6, 7, 71, 83, 85 

fusion for reducing power, 

8 3, 85 

Charge (crucible) for silver and 
gold in ores: 
chromite ore, 129 
cupriferous ores, 118, 119 
hematite, 93, 129 
iron oxide, 89, 93, 129 
limestone, 89, 129 
oxides, 88, 89, 129 
roasted ores, 129 
silicious ores, 88, 92, 129, 130 
sulphide ores, 103, 106, 109, no, 

. in, x 39 

Chiddey’s, A., method for assaying 
cyanide solutions, 186 
Chloridizing roast, 245, 293 
Chlorination, barrel, 252 

of gold ores, 245 
Plattner, 245 

Chromite, assay for gold, 129 
Clays, analysis of, 12 
Clays, fusion of, 75 
Cleaning bucking-board, 32 
Cleaning button before weighing, 
58 

Cobalt ores for silver, 45 
Color of cupels after using, 57 
Color of scorifiers after using, 46 
Color of slags, 9, 91 
Color of vapors, 41 
Combination wet and dry method 
for gold, 181 

Combination wet and dry method 
for silver, 54 
Concentration test, 35 
Coning and quartering, 27, 28 
Copper assay for silver, combination 
wet and dry method, 54 
Copper, influence on loss of gold in 
cupelling, 161 
reactions in cupellation, 59 
in scorification, 43 
Copper bars, assay for gold, 180 
assay for silver, 53 


Copper bars, combination wet and 
dry method for gold, 
181 

combination wet and 
dry method for silver, 

54 

Copper matte, 10, 45, 49, 180 

assay for gold, 180 
silver, 49 

scorification effect of 
borax, 51, 52 
scorification effect of 
glass, 51, 52 
scorification effect of 
silica, 51, 52 

Copper ores, assay for copper, 213 
classification, 213 
crucible fusion for sil¬ 
ver, 118 

effect of different re¬ 
agents, 121 
fusion of, 215 
native, 217 
oxide, 217 
purchase of, 218 
roasting of, 215 
scorification, 45, 49 
sulphide, 214 
Cornish method, 28 
Cream of tartar, 6, 71, 216 
Crucible method: 
copper ores, 216 
gold ores, Class I, 129 
n, 131 

II, iron method, 
I 3 I > *33 

n, f, 139 

silver ores, 86 

Class I, 88 

n, 93> io 3 

iron method, 109 
special methods, 118 
tin ores, 219 
> Crucibles, 15 

analyses, 16 
glazing of, 16 
graphite, 16 
Hessian, 16 
number in cask, 17 
properties of, 15 
size of, 16, 17 
using a second time, 16 
Crust on fusion, 85 




INDEX . 


307 


Crystals of litharge, 56, 65 
Cupellation, 55 

of gold, 160 
of silver, 56 
reactions in, 58, 59 
silver losses in, 62 

Cupels, 20 

assay of, 60, 170, 177 
color after using, 57, 63, 161 
drying, 21 
manufacture, 21 
using only once, 22 
Cyanide process: 
alkali wash, 268 
as applied to concentrates, 262 
experimental treatment of ores, 258 
some reactions, 267 
report on, 260 

for treatment of ores, 256, 260 
Cyanide solutions, assay of, 183, 186 
Cyanogen bromide, 275 

Dead roast, 133, 158, 214 
Desulphurizing agents, 7 
Dusting of ores, 117 

Experiment, barrel chlorination, 252 
bottle amalgamation, 
277 

in concentration, 35 
cyanide process, 258 
pan amalgamation, 300 
Plattner process, 245 
roasting an ore, 157 
roasting concentrates, 

. 157 

silver chloridizing roast, 
.293 

with bullion, 200 
with C.P. silver, 59 
Experimental treatment of gold ores, 
276 

“ Fan ” from vanning, 134 
Ferric oxide, 6, 72 

action in fusion, 73 
Ferrous sulphate solution, 251 
Final sample, fineness, 26, 30, 164 
Fine silver bars, 198 
Fire-brick, 12 

fusing-point, 12 
laying, 23^ 

Fire-clays, 12 


Flour, 6, 71, 113, 125, 149 
Fluor-spar, 73, 192 
Fluxes, 8, 66 
Fluxes, active, 89 
Flux mixture, 113, 125, 149 
Free-milling gold ore, 276 

true value, 278 

Free-milling test in ball mill, 279 
Freezing of button in cupelling, 56 
Fuels, 11 

Furnaces, 11, 13, 14 

repairing, 23 

Fusion, how made in pot-furnace, 83, 
9 °, 135 

in muffle, 92, 115, 130, 131, 
142, 193, 194 
in muffle, how made, 193 
Fusion products, 9 

Galena, assay for silver, m 
Glass, 73, 84 

influence on reducing power 
of a substance, 84, 85 
Gold, combination wet and dry 
method, 181 
flashing of, 156 
fusing-point, 127 
how paid for in ores, 156 
in bismuth, 183 
in copper bars, 180 
in copper matte, 180 
in star antimony, 182 
in zinc-box residues, 163 
loss in cupelling, 160, 161, 164 
parting buttons, 151 
precipitation from AuCia, 248, 
250 

effect of impurities upon pre¬ 
cipitation, 251 

separation from Pt and Ir, 
z 57> 231 

solubility in strong nitric acid, 
152, 162 

solubility in nitrous and nitric 
acid combined, 181 
value of grain, 5 

gramme, 5 
ounce, 4 

volatility of, 159, 160 
weighing of, 155 
Gold amalgam, 286 
Gold bullion, 210 

minerals, rich ores, 128 





INDEX. 


308 

Gold ores, 127 

crucible method, 129 
experimental treatment, 
276 

fusion in muffle, 130 
methods of assay, 128 
scorification method, 128 
steps in assay, 128 
Gold-pan amalgamation, 277 
Gold precipitate, cupellation of, 248 
Gold solutions, assay of, 183 
Granulated lead, 8, 80 

correction for silver, 

47 

testing for silver, 80 
Graphite, analysis, 16 
Graphite crucibles, 17 

heating of, 17 
manufacture of, 
18 

Hammer and anvil, 58 
Hard buttons, 42, 91, 197 
Heath, G. L., copper assaying in Lake 
Superior Region, 218 
Hematite, assay for gold, 129 

silver, 93 

Inquartation, 152 
Introduction, 1 
Iridium, 224, 226, 232, 239 
Iridosmium, 224, 231, 232, 242 
Iron, 7, 70 

removal from fusion, 91 
when necessary in fusion, m 
Iron and arseniate of soda, 70 
Iron and arsenite of soda, 71 
Iron and lead silicates, 70, 192, 196 
Iron and lead sulphide, 7, 70, 74,192 
Iron and litharge, 6, 70, 74 
Iron matte, 10, n 
Iron method, 109, 133, 135 

advantages, 135 
disadvantages, 135 
fusion, 135 
reactions, no 
when used, m, 112 
Iron oxide, assay for gold, 129 

silver, 93 
best flux for, 75, 76 
fusion of, 89 

influence on size of lead 
button, 73 


Iron speiss, 10, 137, 138 

Jewellers’ sweeps, 45 

Labelling samples, 25 * 

Lead, 190 

amount required in scorifica¬ 
tion, 45 

assay of ores for, 190 
granulated, 80 

testing for silver,. 
80 

ratio to copper in scorifying, 

5 L 52 

Lead and nitre, 72 

Lead assay, fusion in muffle, 193 

fusion in pot-furnace, 
194 

general remarks, 197 
iron crucible method, 195 
KCN method, 194 
other metals in, 192 
oxide ores, 195 
slags, 195 
sulphide ores, 192 
Lead bullion, 199 
Lead button, size to cupel, 22 

size from scorification, 

. 46, 56 

size from crucible work, 
81, 109, 116 

Lead matte for silver, 45 
Lead ores, 190 

classification, 191 
Lead silicates, 69 

and iron, 69, 192, 196 
and sulphides, 102 
Lead speiss, assay for silver, 45 
Lead sulphide and iron, 6, 74, 192 
Limestone, assay for gold, 129 

silver, 89 

Litharge, 6, 7, 68, 80 

action on metals, 69 

sulphides, 69 

amount absorbed by cupel, 

59 

amount to use with sulphide 
ores, 104, 105 
and carbon, 6, 70, 84 
and iron, 6, 70, 74 
and nitre fusion, 103, 139, 
142 




INDEX. 


3°9 


Litharge and nitre fusion, advan¬ 
tages and disadvantages, 
142 

and sulphides, 7, 69, 70 
assay for silver, 80 
ratio to copper in ore, 121 
silica, and soda, 69 
testing for silver and gold, 
80 

Litharge crystals, 56, 65 
Loss of gold in cupelling, 160 
scorifying, 128 
silver in cupelling, 62 
scorifying, 65 

Lutes, 23 

Magnesia, best flux for, 75 
Manganese binoxide, 6, 72 

influence on 
size of lead 
button, 93 

Manganiferous ores, 45 
Matte, 10, 11 

assay of, 49, 180 
iron, 112 

Mercury, cleaning of, 292 

recovery from solutions, 286 
retorting of, 290 

Metallic copper, assay for silver, 53 
Metallic particles, 30, 31, 32, 33, 34 
Metallic particles forcing through 
sieve, 30 

Metallurgical laboratory: 
experiments and notes, 243 
general directions, 243 
Mixing the sample, 31, 40, 164 
Mortars, 23 

Muffle chloridizing roast, 293 

report on, 
299 

Muffle fusion, 92, 115, 130, 131, 142, 
J 93> J 94 

effect of temperature, 

86, 93, 115 

Muffles, 22 

care of, 23 
life of, 23 
repairing, 24 
setting, 23 
size, 22 

Nickel ores for silver, 45 
in cupellation, 57 


Nickel speiss, 10 
Nitre, 7, 8, 72, 81 
and lead, 68 

and litharge fusion, 103, 139, 
142 

and size of lead button, 79 
and sulphides, 7, 72, 81, 103 
incorrect oxidizing power of, 82 
influence on silver results, 104 
oxidizing power, 81, 82 
Nitric acid, strength in parting, 152 

Organic matter in ores, 116 
Osmium, 224, 227, 240 
Oxide of iron, 6, 72 

influence on size of 
lead button, 73 

Oxide ores: 

crucible assay for silver, 88 
crucible assay for gold, 129, 130 
Oxidizing agents, 6 

definition, 7 

Oxidizing power of nitre, 81, 82 

ores, 72 

incorrect 
value, 82 

Oxygen, 6, 7 

Palladium, 224, 227, 240 
Pan amalgamation, 300 

reactions, 301 
report on, 302 

Panning test, 35 
Parting silver and gold, 151 
Parting with H 2 S 0 4 , 162 
Parting, acids to use, 151 

transferring the gold, 155 
Pellets, calculation, 32 

examples, 33, 34, 35, 3 6 , 37 
Peroxides in slag, 92 
Platinum, 224, 239 

and H 2 S 0 4 , 163, 231 
effect of gold on parting, 
236, 237 

effect of silver on parting, 
232, 237 

qualitative tests, 225 
quantitative analysis, 228 
separation from gold, 157, 
231 ; 

from silver and 
gold, 231 

and silver alloys, 235 




3 IQ 


INDEX . 


Platinum, sources of, 224 

table of solubility, 226 
Platinum bullion, 229, 232 
Platinum group, table of solubility, 
226 

Platinum ores, assay of, 228 
Plattner chlorination, 245 

report, 249 

Poisoning by KCN, 269 
Potassium carbonate, 66 
Potassium cyanide, 6, 270 

poisoning by, 269 
titration of solu¬ 
tion, 271 

Potassium nitrate, 7, 8, 72, 81 

and metallic lead, 
72 

and sulphides, 7, 
72, 81 

Preliminary fusion, 94 

effect of silica, 96 
borax,96 
soda, 97, 
98, 99 
tempera- 
t u r e, 
101 

Pyrrhotite, assay of, 123 

Quartation, 152 
Quartering, 28, 29 

Reactions in scorification, 43 
Reagents, 5, 66 

testing of, 79 

Recovery of mercury from solutions, 
286 

silver from solutions, 209 
Reducing agents, 5, 83 

definition, 5 

Reducing power, determination of, 83 
influence of silica, 
. 8 4 

influence of differ¬ 
ent reagents, 120 
of ores, 94 

true, 94 
working, 94 
true value, 81 

Refining flux for copper, 217 
Refractories, 12 
Regulus, 10 
Repairing furnaces, 24 


Repairing muffles, 24 
Rescorifying buttons, 46 
Results, closeness of, in ores, 156 
reporting, 58 
Retorting mercury, 290 
Rhodium, 224, 227, 233 
Roast, chloridizing, 245 
dead, 133, 214 
sulphatizing, 245 
Roasted ore, assay for gold, 129 

treatment by bromine, 
272 

Roasting, 244, 246 

an ore, 131, 157 

reactions, 133, 215 
Roasting and reaction process, 245 
Rolling the sample, 31, 40 
Ruthenium, 224, 227, 242 
oxides, 242 

Salt, 73 

Sample, final one, 30 

fineness of, 26, 30, 164 
labelling, 25 

mixing and rolling, 30, 40 
Sampling, 25 

methods, 27 
Cornish method, 28 
necessity of cleaning ma¬ 
chines, 26, 32 
Scorification, reactions, 43 
Scorification method: 
for gold ores, 128, 131 
for silver ores, 39 
fusion period, 41 
liquefaction period, 42 
roasting period, 41 
scorification period, 41 
Scorifiers, 19 

addition of Si 0 2 to, 20 
color after using, 46 
effect of diameter on size 
of lead button, 46 
size, 19 

Silica, 73, 84, 85 

addition to a fusion, 106 
and litharge, 69 
influence on the reducing 
power of a substance, 84, 
. 85,96 

in scorification, 44 
safe ratio to soda, 108 
soda, and litharge, 69 




INDEX. 


3n 


Silicates of soda, 67 
Silicious ore: 

assay for gold, 129, 130 
assay for silver, 88, 92 
Silver, assay of solutions, 183 

combination wet and dry 
method, 54 

effect of increasing ratio to 
gold on gold loss, 162 
experiment with C.P., 59 
fusing-point, 39 
in bismuth, 183 
star antimony, 182 
recovery from solutions, 209, 
286 

Silver amalgam, 284, 285 
Silver and copper alloys, cupella- 
tion of, 204 

Silver bead, weighing of, 58 

of unusual appearance, 
65 

Silver bullion, 198, 201, 205 
Silver button, blicking, 56, 61 
cleaning, 58 

Silver chloridizing roast, 293 

report 00,299 

Silver loss due to nitre, 114 
Silver losses in cupelling, 58, 59, 61, 

65 

influence of bone-ash, 61 
cupel, 61 
copper, 64, 65 
lead, 64 
tellurium, 65 
temperature, 62 
Silver ores, assay of, 39 

crucible method, 66 
pan amalgamation, 300 
scorification method, 39 
with small amount of 
sulphides, 106 
Size of lead buttons, 116 
Slag, 9, 10 

assay of, 170, 177, 195 
effect of temperature on color, 
10 

Soda, litharge, and silica, 69 
Sodium carbonate, 66 

and metallic sul¬ 
phides, 67, 98 
influence on re¬ 
ducing power, 
84, 97 


Sodium nitrate, 7, 72 
Sodium silicates, 67 
Solutions, assay of, 183 

making up of, 243 
Spatula, for taking samples, 30 
Special methods, 118 
gold in zinc-box residues, 163 
silver in antimonial ores, 122 
copper ores, 118 
Speiss, 10, 11 

influence of temperature on 
formation, 137 
iron, 10 
lead, 45 
nickel, 10 

Sperrylite, 225, 228 

Spitting of ores, 48 

Split shovel, 29 

Sprouting of button, 56, 57 

Stamp-mill work, 283 

Stanniferous ores, assay for silver, 45 

Starch, 6, 71 

Sterling silver, 5 

Sulphates, action in presence of 
litharge, 78 

Sulphates, decomposition by heat, 133 
order of decomposition by 
heat, 294 

roasted-ore test, 268 
Sulphatizing roast, 245 
Sulphides and lead silicates, 102 
bicarb, soda, 67, 98 
Sulphide ores, assay for silver, 103, 

106, no, 
in, 112 
gold, 131, 
i33> i39 

Sulphur, 8 

Sulphuric acid, parting with, 162 
Sulphurizing agents, 8 
Surcharge, 162 

Telluride ores, assay of, 143 
Tellurium, effect on gold in cupella- 
tion, 150 
test for, 149 

Temperature, effect of insufficient, in 
muffle fusion, 86, 93, 

115 

influence on color of 
slag, 9, 10 

influence on cupella- 
tion of gold, 160 


l 




3 12 


INDEX 


Temperature, influence on cupella- 
tion of silver, 62 
influence on the for¬ 
mation of speiss, 137 
influence on* size of 
lead button, 86,101, 
116 

Testing a roasted ore for sulphates, 
268 

Testing-reagents, 79 

R.P. of reagents, 83 
Time of fusion, 90, 94, 108, 136, 193, 
215, 222 
Tin, 219 

effect of Si 0 2 on fusion, 220 
methods of assay, 221, 222 
steps in assay, 220 
Tin ores, 219 
Tongs, annealing-cup, 155 
crucible, 14 
cupel, 14 
lead assay, 193 
scorifier, 14, 41 
True R.P. of an ore, 81, 94 

Van Liew’s wet method for gold, 181 

silver, 54 


Vanning shovel, 87, 133, 281 
Vanning test, 35 

for character of ore, 134 
Vapors, color from scorification, 41 
VolharcTs wet method for bullion, 207 
Volumetric method for silver, 199, 207 


Washing test for character of ore, 134 
Weighing silver beads, 58 

and gold beads, 150 

Weights, 2 

memoranda, 4 
White flux, 194 

Working R.P. of an ore, 81, 94 


Zinc blende, assay of, 45, 108 
Zinc sulphate, 78 

Zinc-box residues, assay for gold, 163 

silver, 163 
best charge for 
scorification, 171, 
180 

Zinc, reactions in cupellation, 59 

scorification, 44 



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Chemistry. . .. 



1 

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27 cents additional.).8vo, 3 50 

Comstock’s Field Astronomy for Engineers.8vo, 2 50 

Davis’s Elevation and Stadia Tables.8vo, 1 00 

Elliott’s Engineering for Land Drainage. i2mo, 1 50 

Practical Farm Drainage.i2mo, 1 00 

*Fiebeger’s Treatise on Civil Engineering.8vo, 5 00 

Folwell’s Sewerage. (Designing and Maintenance.).8vo, 3 00 

Freitag’s Architectural Engineering. 2d Edition, Rewritten.8vo, 350 

French and I zes’s Stereotomy.8vo, 2 50 

Goodhue's Municipal Improvements.i2mo, 1 75 

Goodrich’s Economic Disposal of Towns’ Refuse.8vo, 3 50 

Gore’s Elements of Geodesy. 8vo, 2 50 

Hayford’s Text-book of Geodetic Astronomy.8vo, 3 00 

Hering’s Ready Reference Tables (Conversion Factors).i6mo, morocco, 2 50 

Howe’s Retaining Walls for Earth.i2mo, 1 25 

Johnson’s (J. B.) Theory and Practice of Surveying.Small 8vo, 4 00 

Johnson’s (L. J.) Statics by Algebraic and Graphic Methods.8vo, 2 00 

Laplace’s Philosophical Essay on Probabilities. (Truscoit and Emory.). i2mo, 2 00 

Mahan’s Treatise on Civil Engineering. (1873.) (V/ood.;..8vo, 500 

* Descriptive Geometry.8vo, 1 50 

Merriman’s Elements of Precise Surveying and Geodesy. ^vc, 2 50 

Merriman and Brooks’s Handbook for Surveyors.i6mo, moro„ " 00 

Nugent’s Plane Surveying.8vo, 3 5^ 

Ogden’s Sewer Design.i2mo, 2 00 

Patton’s Treatise on Civil Engineering.8vo half leather, 7 50 

Reed’s Topographical Drawing and Sketching.4to, 5 00 

Rideal’s Sewage and the Bacterial Purification of Sewage.8vo, 3 50 

Siebert and Biggin’s Modern Stone-cutting and Masonry.8vo, 1 50 

Smith’s Manual of Topographical Drawing. (McMillan.).8vo, 2 50 

Sondericker’s Graphic Statics, with Applications to Trusses, Beams, and Arches. 

8vo, 2 00 

Taylor and Thompson’s Treatise on Concrete, Plain and Reinforced.8vo, 5 00 

* Trautwine’s Civil Engineer’s Pocket-book.i6mo, morocco, 5 00 

Wait’s Engineering and Architectural Jurisprudence.8vo, 6 00 

Sheep, 6 50 

Law of Operations Preliminary to Construction in Engineering and Archi¬ 
tecture.8vo, 5 00 

Sheep, 5 50 

Law of Contracts.8vo, 3 00 

Warren’s Stereotomy—Problems in Stone-cutting.8vo, 2 50 

Webb’s Problems in the Use and Adjustment of Engineering Instruments. 

i6mo, morocco, 1 25 

Wilson’s Topographic Surveying.8vo, 3 50 


BRIDGES AND ROOFS. 

Boiler’s Practical Treatise on the Construction of Iron Highway Bridges. .8vo, 2 00 

* Thames River Bridge.4to, paper, 5 00 

Burr’s Course on the Stresses in Bridges and Roof Trusses, Arched Ribs, and 

Suspension Bridges.8vo, 3 50 


6 









































Burr and Falk’s Influence Lines for Bridge and Roof Computations. . . .8vo, 3 00 

Design and Construction of Metallic Bridges.8vo, 5 00 

Du Bois’s Mechanics of Engineering. Vol. II.Small 4to, 10 00 

Foster’s Treatise on Wooden Trestle Bridges.4to, 5 00 

Fowler’s Ordinary Foundations.8vo, 3 50 

Greene’s Roof Trusses.;.8vo, 1 25 

Bridge Trusses.8vo, 2 50 

Arches in Wood, Iron, and Stone.8vo, 2 50 

Howe’s Treatise on Arches.8vo, 4 00 

Design of Simple Roof-trusses in Wood and Steel.8vo, 2 00 

Johnson, Bryan, and Turneaure’s Theory and Practice in the Designing of 

Modern Framed Structures.Small 4to, 10 00 

Merriman and Jacoby’s Text-book on Roofs and Bridges: 

Part I. Stresses in Simple Trusses.8vo, 2 50 

Part II. Graphic Statics.8vo, 2 50 

Part III. Bridge Design. . .. 8vo, 2 50 

Part IV. Higher Structures.8vo, 2 50 

Morison’s Memphis Bridge.4to, 10 00 

Waddell’s De Pontibus, a Pocket-book for Bridge Engineers. . i6mo, morocco, 2 00 

Specifications for Steel Bridges.i2mo, 1 25 

Wright’s Designing of Draw-spans. Two parts in one volume.8vo, 3 50 

HYDRAULICS. 

Bazin’s Experiments upon the Contraction of the Liquid Vein Issuing from 

an Orifice. (Trautwine.).8vo, 2 00 

Bovey’s Treatise on Hydraulics.8vo, 5 00 

Church’s Mechanics of Engineering.8vo, 6 00 

Diagrams of Mean Velocity of Water in Open Channels.paper, 1 50 

Hydraulic Motors.8vo, 2 00 

Coffin’s Graphical Solution of Hydraulic Problems.i6mo, morocco, 2 50 

Flather’s Dynamometers, and the Measurement of Power.i2mo, 3 00 

Folwell’s Water-supply Engineering.8vo, 4 00 

Frizell’s Water-power. .8vo, 5 00 

Fuertes’s Water and Public Health.i2mo, 1 50 

Water-filtration Works.i2mo, 2 50 

Ganguillet and Kutter’s General Formula for the Uniform Flow of Water in 

Rivers and Other Channels. (Hering and Trautwine.).8vo, 4 00 

Hazen’s Filtration of Public Water-supply.8vo, 3 00 

Hazlehurst’s Towers and Tanks for Water-works.8vo, 2 50 

Herschel’s 115 Experiments on the Carrying Capacity of Large, Riveted, Metal 

Conduits.8vo, 2 00 

Mason’s Water-supply. (Considered Principally from a Sanitary Standpoint.) 

8vo, 4 00 

Merriman’s Treatise on Hydraulics.8vo, 5 00 

* Michie’s Elements of Analytical Mechanics.8vo, 4 00 

Schuyler’s Reservoirs for Irrigation, Water-power, and Domestic Water- 

supply.Large 8vo, 5 00 

** Thomas and Watt’s Improvement of Rivers. (Post., 44c. additional.).4to, 6 00 

Turneaure and Russell’s Public Water-supplies.:.8vo, 5 00 

Wegmann’s Design and Construction of Dams.4to, 5 00 

Water-supply of the City of New York from 1658 to 1895.4to, 10 00 

Williams and Hazen’s Hydraulic Tables.8vo, 1 50 

Wilson’s Irrigation Engineering.Small 8vo, 4 00 

Wolff’s Windmill as a Prime Mover.8vo, 3 00 

Wood’s Turbines.8vo, 250 

Elements of Analytical Mechanics.8vo, 3 00 


7 













































MATERIALS OF ENGINEERING. 


Baker’s Treatise on Masonry Construction.8vo, 5 00 

Roads and Pavements.8vo, 5 00 

Black’s United States Public Works..Oblong 4to, 5 00 

* Bovey’s Strength of Materials and Theory of Structures.8vo, 7 50 

Burr’s Elasticity and Resistance of the Materials of Engineering.8vo, 7 50 

Byrne’s Highway Construction.8vo, 5 00 

Inspection of the Materials and Workmanship Employed in Construction. 

i6mo, 3 00 

Church’s Mechanics of Engineering.8vo, 6 00 

Du Bois’s Mechanics of Engineering. Vol. I.Small 4to, 7 50 

*Eckel’s Cements, Limes, and Plasters.8vo, 6 00 

Johnson’s Materials of Construction.Large 8vo, 6 00 

Fowler’s Ordinary Foundations.8vo, 3 50 

* Greene’s Structural Mechanics.8vo, 2 50 

Keep's Cast Iron. 8vo, 2 50 

Lanza’s Applied Mechanics.8vo, 7 50 

Marten’s Handbook on Testing Materials. (Henning.) 2 vols.8vo, 7 50 

Maurer’s Technical Mechanics.8vo, 4 00 

Merrill’s Stones for Building and Decoration. 8vo, 5 00 

Merriman’s Mechanics of Materials.8vo, 5 00 

Strength of Materials.i2mo, 1 00 

Metcalf’s Steel. A Manual for Steel-users.i2mo, 2 00 

Patton’s Practical Treatise on Foundations.8v®, 5 00 

Richardson’s Modern Asphalt Pavements.8vo, 3 00 

Richey’s Handbook for Superintendents of Construction.i6mo, mor., 4 00 

Rockwell’s Roads and Pavements in France.nmo, 1 25 

Sabin’s Industrial and Artistic Technology of Paints and Varnish.8vo, 3 00 

Smith’s Materials of Machines.i2mo, 1 00 

Snow’s Principal Species of Wood.8vo, 3 50 

Spalding’s Hydraulic Cement. nmo, 2 00 

Text-book on Roads and Pavements.nmo, 2 00 

Taylor and Thompson’s Treatise on Concrete, Plain and Reinforced.8vo, 5 00 

Thurston’s Materials of Engineering. 3 Parts.8vo, 8 00 

Parti. Non-metallic Materials of Engineering and Metallurgy.8vo, 2 00 

Part II. Iron and Steel.8vo, 3 50 

Part III. A Treatise on Brasses, Bronzes, and Other Alloys and their 

Constituents.8vo, 2 50 

Thurston’s Text-book of the Materials of Construction.8vo, 5 00 

Tillson’s Street Pavements and Paving Materials.8vo, 4 00 

Waddell’s De Pontibus. (A Pocket-book for Bridge Engineers.). . i6mo, mor., 2 00 

Specifications for Steel Bridges.i2mo, 1 25 

Wood’s (De V.) Treatise on the Resistance of Materials, and an Appendix on 

the Preservation of Timber.8vo, 2 00 

Wood’s (De V.) Elements of Analytical Mechanics.8vo, 3 00 

Wood’s (M. P.) Rustless Coatings: Corrosion and Electrolysis of Iron and 

Steel.8vo, 4 00 


RAILWAY ENGINEERING. 

Andrew’s Handbook for Street Railway Engineers.3x5 inches, morocco, 1 25 

Berg’s Buildings and Structures of American Railroads.4to, 5 00 

Brook’s Handbook of Street Railroad Location.i6mo, morocco, 1 50 

Butt’s Civil Engineer’s Field-book.i6mo, morocco, 2 50 

Crandall’s Transition Curve.i6mo, morocco, 1 50 

Railway and Other Earthwork Tables.8vo, 1 50 

Dawson’s “Engineering” and Electric Traction Pocket-book.. i6mo, morocco, 5 00 

8 
















































Dredge’s History of the Pennsylvania Railroad: (1879).Paper, 5 00 

* Drinker’s Tunnelling, Explosive Compounds, and Rock Drills.4to, half mor., 25 00 

Fisher’s Table of Cubic Yards.Cardboard, 25 

Godwin’s Railroad Engineers’ Field-book and Explorers’ Guide. . . i6mo, mor., 2 50 

Howard’s Transition Curve Field-book.i6mo, morocco, 1 50 

Hudson’s Tables for Calculating the Cubic Contents of Excavations and Em¬ 
bankments.8vo, 1 00 

Molitor and Beard’s Manual for Resident Engineers.i6mo, 1 00 

Nagle’s Field Manual for Railroad Engineers.i6mo, morocco, 3 00 

Philbrick’s Field Manual for Engineers.i6mo, morocco, 3 00 

Searles’s Field Engineering.i6mo, morocco, 3 00 

Railroad Spiral.i6mo, morocco, 1 50 

Taylor’s Prismoidal Formulae and Earthwork.8vo, 1 50 

* Trautwine’s Method of Calculating the Cube Contents of Excavations and 

Embankments by the Aid of Diagrams.8vo, 2 00 

The Field Practice of Laying Out Circular Curves for Railroads. 

i2mo, morocco, 2 50 

Cross-section Sheet.Paper, 25 

Webb’s Railroad Construction.i6mo, morocco, 5 00 

Wellington’s Economic Theory of the Location of Railways.Small 8vo, 5 00 


DRAWING. 


Barr’s Kinematics of Machinery.8vo, 2 50 

* Bartlett’s Mechanical Drawing.8vo, 3 00 

* “ “ “ Abridged Ed.8vo, 1 50 

Coolidge’s Manual of Drawing.8vo, pap^r 1 00 

Coolidge and Freeman’s Elements of General Drafting for Mechanical Engi¬ 
neers.Oblong 4to, 2 §o 

Durley’s Kinematics of Machines.8vo, 4 00 

Emch’s Introduction to Projective Geometry and its Applications.8vo, 2 50 

Hill’s Text-book on Shades and Shadows, and Perspective.8vo, 2 00 

Jamison’s Elements of Mechanical Drawing.8vo, 2 50 

Advanced Mechanical Drawing...8vo, 2 00 

Jones’s Machine Design: 

Part I. Kinematics of Machinery.8vo, 1 50 

Part II. Form, Strength, and Proportions of Parts.8vo, 3 00 

MacCord’s Elements of Descriptive Geometry.8vo, 3 00 

Kinematics; or, Practical Mechanism.8vo, 5 00 

Mechanical Drawing.4to, 4 00 

Velocity Diagrams.8vo, 1 50 

MacLeod’s Descriptive Geometry.Small 8vo, 1 50 

* Mahan’s Descriptive Geometry and Stone-cutting.8vo, 1 50 

Industrial Drawing. (Thompson.).8vo, 3 50 

Moyer’s Descriptive Geometry.8vo, 2 00 

Reed’s Topographical Drawing and Sketching.4to, 5 00 

Reid’s Course in Mechanical Drawing.8vo, 2 00 

Text-book of Mechanical Drawing and Elementary Machine Design.8vo, 3 00 

Robinson’s Principles of Mechanism.8vo, 3 00 

Schwamb and Merrill’s Elements of Mechanism.8vo, 3 co 

Smith’s (R. S.) Manual of Topographical Drawing. (McMillan.).8vo, 2 50 

Smith (A. W.) and Marx’s Machine Design. 8vo, 3 00 

Warren’s Elements of Plane and Solid Free-hand Geometrical Drawing. i2mo, 1 00 

Drafting Instruments and Operations.i2mo, 1 25 

Manual of Elementary Projection Drawing.i2mo, 1 50 

Manual of Elementary Problems in the Linear Perspective of Form and 

Shadow.i2mo, 1 00 

Plane Problems in Elementary Geometry.nmo, 1 25 

9 














































Warren’s Primary Geometry.i2mo, 75 

Elements of Descriptive Geometry, Shadows, and Perspective.8vo, 3 50 

General Problems of Shades and Shadows.8vo, 3 00 

Elements of Machine Construction and Drawing.8vo, 7 5 ° 

Problems, Theorems, and Examples in Descriptive Geometry.8vo, 2 50 

Weisbach’s Kinematics and Power of Transmission. (Hermann and 

Klein.). 8vo, 5 o 0 

Whelpley’s Practical Instruction in the Art of Letter Engraving.nmo, 2 00 

Wilson’s (H. M.) Topographic Surveying.8vo, 3 50 

Wilson’s (V. T.) Free-hand Perspective.8vo, 2 50 

Wilson’s (V. T.) Free-hand Lettering.8vo, 1 00 

Woolf’s Elementary Course in Descriptive Geometry.Large 8vo, 3 00 


ELECTRICITY AND PHYSICS. 

Anthony and Brackett’s Text-book of Physics. (Magie.).Small 8vo, 

Anthony’s Lecture-notes on the Theory of Electrical Measurements. .. . nmo, 

Benjamin’s History of Electricity.8vo, 

Voltaic Cell.8vo, 

Classen’s Quantitative Chemical Analysis by Electrolysis. (Boltwood.).8vo, 

Crehore and Squier’s Polarizing Photo-chronograph.8vo, 

Dawson’s “Engineering” and Electric Traction Pocket-book. i6mo, morocco, 
Dolezalek’s Theory of the Lead Accumulator (Storage Battery). (Von 

Ende.).nmo, 

Duhem’s Thermodynamics and Chemistry. (Burgess.).8vo, 

Flather’s Dynamometers, and the Measurement of Power.nmo, 

Gilbert’s De Magnete. (Mottelay.).8vo, 

Hanchett’s Alternating Currents Explained.nmo, 

Hering’s Ready Reference Tables (Conversion Factors).i6mo, morocco, 

Holman’s Precision of Measurements.8vo, 

Telescopic Mirror-scale Method, Adjustments, and Tests. . . .Large 8vo, 

Xinzbrunner’s Testing of Continuous-current Machines.8vo, 

Landauer’s Spectrum Analysis. (Tingle.).8vo, 

Le Chatelier s High-temperature Measurements. (Boudouard—Burgess.) nmo, 
Lob’s Electrochemistry of Organic Compounds. (Lorenz.).8vo, 

* Lyons’s Treatise on Electromagnetic Phenomena. Vols. I. and II. 8vo, each, 

* Michie’s Elements of Wave Motion Relating to. Sound and Light.8vo, 

Niaudet’s Elementary Treatise on Electric Batteries. (Fishback.).nmo, 

* Rosenberg’s Electrical Engineering. (Haldane Gee—Kinzbrunner.). . .8vo, 

Ryan, Norris, and Hoxie’s Electrical Machinery. Vol. 1 .8vo, 

Thurston’s Stationary Steam-engines.8vo, 

* Tillman’s Elementary Lessons in Heat.8vo, 

Tory and Pitcher’s Manual of Laboratory Physics.Small 8vo, 

Ulke’s Modern Electrolytic Copper Refining.8vo, 


LAW. 


* Davis’s Elements of Law.8vo, 

* Treatise on the Military Law of United States.8vo, 

* Sheep, 

Manual for Courts-martial.i6mo, morocco, 

Wait’s Engineering and Architectural Jurisprudence.8vo, 

Sheep, 

Law of Operations Preliminary to Construction in Engineering and Archi¬ 
tecture, .8vo 

Sheep, 

Law of Contracts.8vo, 

Winthrop’s Abridgment of Military Law.nmo, 


3 00 

1 00 
3 00 
3 00 
3 00 

3 00 

5 00 

2 50 

4 00 

3 00 
2 50 

1 00 

2 50 
2 00 

75 

2 00 

3 00 
3 00 

3 op 

6 00 

4 00 
2 50 

1 50 

2 50 
2 50 

1 50 

2 OO 

3 00 


2 50 
7 00 
7 50 

1 50 
6 00 
6 so 

5 00 
5 50 

3 00 

2 50 


10 










































MANUFACTURES. 


Bernadou’s Smokeless Powder—Nitro-cellulose and Theory of the Cellulose 

Molecule.nmo, 

Bolland’s Iron Founder.i2mo, 

“The Iron Founder,” Supplement.i2mo, 

Encyclopedia of Founding and Dictionary of Foundry Terms Used in the 

Practice of Moulding.i2mo, 

Eissler’s Modern High Explosives.8vo, 

Effront’s Enzymes and their Applications. (Prescott.).8vo, 

Fitzgerald’s Boston Machinist. nmo, 

Ford’s Boiler Making for Boiler Makers.i8mo, 

Hopkin’s Oil-chemists’ Handbook.8vo, 

Keep’s Cast Iron.8vo, 

Leach’s The Inspection and Analysis of Food with Special Reference to State 

Control.Large 8vo, 

Matthews’s The Textile Fibres.8vo, 

Metcalf’s Steel. A Manual for Steel-users.nmo, 

Metcalfe’s Cost of Manufactures—And the Administration of Workshops.8vo, 

Meyer’s Modern Locomotive Construction.4to, 

Morse’s Calculations used in Cane-sugar Factories.i6mo, morocco, 

* Reisig’s Guide to Piece-dyeing.8vo, 

Sabin’s Industrial and Artistic Technology of Paints and Varnish.8vo, 

Smith’s Press-working of Metals.8vo, 

Spalding’s Hydraulic Cement.nmo, 

Spencer’s Handbook for Chemists of Beet-sugar Houses.i6mo, morocco, 

Handbook for Cane Sugar Manufacturers.i6mo, morocco, 

Taylor and Thompson’s Treatise on Concrete, Plain and Reinforced.8vo, 

Thurston’s Manual of Steam-boilers, their Designs, Construction and Opera¬ 
tion.8vo, 

* Walke’s Lectures on Explosives.8vo, 

Ware’s Beet-sugar Manufacture and Refining.Small 8vo, 

West’s American Foundry Practice.nmo, 

Moulder’s Text-book.nmo, 

Wolff’s Windmill as a Prime Mover.8vo, 

Wood’s Rustless Coatings: Corrosion and Electrolysis of Iron and Steel. .8vo, 


2 50 
2 50 
2 50 

3 OO 

4 oo 
3 oo 
i oo 

1 oo 
3 oo 

2 50 

7 50 
3 50 

2 OO 

5 oo 
10 oo 

I 50 
25 oo 

3 oo 
oo 
oo 
oo 
oo 
oo 


5 

4 

4 

2 

2 

3 

4 


oo 

oo 

00 

50 

50 

oo 

oo 


MATHEMATICS. 


Baker’s Elliptic Functions.8vo, i 50 

* Bass’s Elements of Differential Calculus.nmo, 4 00 

Briggs’s Elements of Plane Analytic Geometry.nmo, 1 00 

Compton’s Manual of Logarithmic Computations.nmo, 1 50 

Davis’s Introduction to the Logic of Algebra.8vo, 1 50 

* Dickson’s College Algebra.Large nmo, 1 50 

* Introduction to the Theory of Algebraic Equations.Large nmo, 1 25 

Emch’s Introduction to Projective Geometry and its Applications.8vo, 2 50 

Halsted’s Elements of Geometry.8vo, 1 75 

Elementary Synthetic Geometry.8vo, 1 50 

Rational Geometry.i2mo, 1 75 


* Johnson’s (J. B.) Three-place Logarithmic Tables: Vest-pocket size.paper, 15 

100 copies for 5 00 

* Mounted on heavy cardboard, 8X10 inches, 25 

10 copies for 2 00 

Johnson’s (W. W.) Elementary Treatise on Differential Calculus. .Small 8 vo, 3 00 
Johnson’s (W W.) Elementary Treatise on the Integral Calculus.Small 8 vo, 1 50 

11 









































Johnson’s (W. W.) Curve Tracing in Cartesian Co-ordinates.nmo, 

Johnson’s (W. W.) Treatise on Ordinary and Partial Differential Equations. 

Small 8vo, 

Johnson’s (W. W.) Theory of Errors and the Method of Least Squares, nmo, 

* Johnson’s (W. W.) Theoretical Mechanics.nmo, 

Laplace’s Philosophical Essay on Probabilities. (Truscott and Emory.), nmo, 

* Ludlow and Bass. Elements of Trigonometry and Logarithmic and Other 

Tables.8vo, 

Trigonometry and Tables published separately.Each, 

♦Ludlow’s Logarithmic and Trigonometric Tables.8vo, 

Mathematical Monographs. Edited by Mansfield Merriman and Robert 

S. Woodward...Octavo, each 

No. x. History of Modern Mathematics, by David Eugene Smith. 

No. 2. Synthetic Projective Geometry, by George Bruce Halsted. 

No. 3. Determinants, by Laenas Gifford Weld. No. 4. Hyper¬ 
bolic Functions, by James McMahon. No. 5. Harmonic Func¬ 
tions, by William E. Byerly. No. 6. Grassmann’s Space Analysis, 
by Edward W. Hyde. No. 7. Probability and Theory of Errors, 
by Robert S. Woodward. No. 8. Vector Analysis and Quaternions, 
by Alexander Macfarlane. No. 9. Differential Equations, by 
William Woolsey Johnson. No. 10. The Solution of Equations, 
by] Mansfield Merriman. No. 11. Functions of a Complex Variable, 


by Thomas S. Fiske. 

Maurer’s Technical Mechanics.8vo, 

Merriman and Woodward’s Higher Mathematics.8vo, 

Merriman’s Method of Least Squares.8vo, 

Rice and Johnson’s Elementary Treatise on the Differential Calculus.. Sm. 8vo, 

Differential and Integral Calculus. 2 vols. in one.Small 8vo, 

Wood’s Elements of Co-ordinate Geometry.8vo, 

Trigonometry: Analytical, Plane, and Spherical.i2mo, 


1 00 

3 50 

1 50 
3 00 

2 00 

3 00 
2 00 
1 00 

1 00 


4 00 

5 00 

2 00 

3 00 
2 50 
2 00 
1 00 


MECHANICAL ENGINEERING. 

MATERIALS OF ENGINEERING, STEAM-ENGINES AND BOILERS. 


Bacon’s Forge Practice.i2mo, 1 50 

Baldwin’s Steam Heating for Buildings.nmo, 2 50 

Barr’s Kinematics of Machinery.8vo, 2 50 

* Bartlett’s Mechanical Drawing.8vo, 3 00 

* “ “ “ Abridged Ed.8vo, 1 50 

Benjamin’s Wrinkles and Recipes.v..nmo, 2 00 

Carpenter’s Experimental Engineering.8vo, 6 00 

Heating and Ventilating Buildings.8vo, 4 00 

Cary’s Smoke Suppression in Plants using Bituminous Coal. (In Prepara¬ 
tion.) 

Clerk’s Gas and Oil Engine.Small 8vo, 4 00 

Coolidge’s Manual of Drawing.8vo, paper, 1 00 

Coolidge and Freeman’s Elements of General Drafting for Mechanical En¬ 
gineers.Oblong 4to, 2 50 

Cromwell’s Treatise on Toothed Gearing.i2mo, 1 50 

Treatise on Belts and Pulleys.nmo, 1 50 

Durley’s Kinematics of Machines.8vo, 4 00 

Flather’s Dynamometers and the Measurement of Power.i2mo, 3 00 

Rope Driving.12mo, 2 00 

Gill’s Gas and Fuel Analysis for Engineers.i2mo, 1 25 

Hall’s Car Lubrication.i2mo, 1 00 

Hering’s Ready Reference Tables (Conversion Factors).i6mo, morocco, 2 50 

12 

































Hutton’s The Gas Engine.8vo, 5 00 

Jamison’s Mechanical Drawing.8vo, 2 50 

Jones’s Machine Design: 

Part I. Kinematics of Machinery.8vo, 1 50 

Part II. Form, Strength, and Proportions of Parts.8vo, 3 00 

Kent’s Mechanical Engineers’Pocket-book.i6mo, morocco, 5 00 

Kerr’s Power and Power Transmission.8vo, 2 00 

Leonard’s Machine Shop, Tools, and Methods.8vo, 4 00 

* Lorenz’s Modern Refrigerating Machinery. (Pope, Haven, and Dean.) . . 8vo, 4 00 

MacCord’s Kinematics; or, Practical Mechanism.8vo, 5 00 

Mechanical Drawing.4to, 4 00 

Velocity Diagrams.8vo, 1 50 

MacFarland’s Standard Reduction Factors for Gases.8vo, 1 50 

Mahan’s Industrial Drawing. (Thompson.).8vo, 3 50 

Poole’s Calorific Power of Fuels.8vo, 3 00 

Reid’s Course in Mechanical Drawing.8vo, 2 00 

Text-book of Mechanical Drawing and Elementary Machine Design.8vo, 3 00 

Richard’s Compressed Air.nmo, 1 50 

Robinson’s Principles of Mechanism.8vo, 3 00 

Schwamb and Merrill’s Elements of Mechanism.8vo, 3 00 

Smith’s (O.) Press-working of Metals.8vo, 3 00 

Smith (A. W.) and Marx’s Machine Design.8vo, 3 00 

Thurston’s Treatise on Friction and Lost Work in Machinery and Mill 

Work.8vo, 3 00 

Animal as a Machine and Prime Motor, and the Laws of Energetics. nmo, 1 00 

Warren’s Elements of Machine Construction and Drawing. 8 vo, 7 50 

Weisbach’s Kinematics and the Power of Transmission. (Herrmann— 

Klein.).8vo, 5 00 

Machinery of Transmission and Governors. (Herrmann—Klein.). .8vo, 5 00 

Wolff’s Windmill as a Prime Mover.8vo, 3 00 

Wood’s Turbines..8vo, 2 50 

MATERIALS OP ENGINEERING. 

* Bovey’s Strength of Materials and Theory of Structures.8vo, 7 50 

Burr’s Elasticity and Resistance of the Materials of Engineering. 6th Edition. 

Reset. 8vo, 7 50 

Church’s Mechanics of Engineering.8vo, 6 00 

* Greene’s Structural Mechanics.8vo, 2 50 

Johnson’s Materials of Construction.8vo, 6 00 

Keep’s Cast Iron.8vo, 2 50 

Lanza’s Applied Mechanics.8vo, 7 50 

Martens’s Handbook on Testing Materials. (Henning.).8vo, 7 50 

Maurer’s Technical Mechanics.8vo, 4 00 

Merriman’s Mechanics of Materials.8vo, 5 00 

Strength of Materials.nmo, 1 00 

Metcalf’s Steel. A manual for Steel-users.nmo, 2 00 

Sabin’s Industrial and Artistic Technology of Paints and Varnish.8vo, 3 00 

Smith’s Materials of Machines.nmo, 1 00 

Thurston’s Materials of Engineering.3 vols., 8vo, 8 00 

Part II. Iron and Steel.8vo, 3 50 

Part III. A Treatise on Brasses, Bronzes, and Other Alloys and their 

Constituents.8vo, 2 50 

Text-book of the Materials of Construction.8vo, 5 00 

Wood’s (De V.) Treatise on the Resistance of Materials and an Appendix on 

the Preservation of Timber.8vo, 2 00 


13 















































Wood’s (De V.) Elements of Analytical Mechanics.8vo, 3 00 

Wood’s (M. P.) Rustless Coatings: Corrosion and Electrolysis of Iron and 

Steel. 8vo, 4 00 

STEAM-ENGINES AND BOILERS. 

Berry’s Temperature-entropy Diagram.i2mo, 1 25 

Carnot’s Reflections on the Motive Power of Heat. (Thurston.).i2mo, 1 50 

Dawson’s “Engineering” and Electric Traction Pocket-book. . i6mo, mor., 5 00 

Ford’s Boiler Making for Boiler Makers.i8mo, 1 00 

Goss’s Locomotive Sparks.8vo, 2 00 

Hemenway’s Indicator Practice and Steam-engine Economy.i2mo, 2 00 

Hutton’s Mechanical Engineering of Power Plants.8vo, 5 00 

Heat and Heat-engines.8vo, 5 00 

Kent’s Steam boiler Economy.8vo, 4 00 

Kneass’s Practice and Theory of the Injector.8vo, 1 50 

MacCord’s Slide-valves.8vo, 2 00 

Meyer’s Modern Locomotive Construction.4to, 10 00 

Peabody’s Manual of the Steam-engine Indicator.i2mo. 1 50 

Tables of the Properties of Saturated Steam and Other Vapors .8vo, 1 00 

Thermodynamics of the Steam-engine and Other Heat-engines.8vo, 5 00 

Valve-gears for Steam-engines.8vo, 2 50 

Peabody and Miller’s Steam-boilers.8vo, 4 00 

Pray’s Twenty Years with the Indicator.Large 8vo, 2 50 

Pupin’s Thermodynamics of Reversible Cycles in Gases and Saturated Vapors. 

(Osterberg.).i2mo, 1 25 

Reagan’s Locomotives: Simple Compound, and Electric.i2tno, 2 50 

Rontgen’s Principles of Thermodynamics. (Du Bois.).8vo, 5 00 

Sinclair’s Locomotive Engine Running and Management.i2mo, 2 00 

Smart’s Handbook of Engineering Laboratory Practice.i2mo, 2 50 

Snow’s Steam-boiler Practice.8vo, 3 00 

Spangler’s Valve-gears.8vo, 2 50 

Notes on Thermodynamics.i2mo, 1 00 

Spangler, Greene, and Marshall’s Elements of Steam-engineering.8vo, 3 00 

Thurston’s Handy Tables.8vo, 1 50 

Manual of the Steam-engine.2 vols., 8vo, 10 00 

Part I. History, Structure, and Theory.8vo, 6 00 

Part II. Design, Construction, and Operation.8vo, 6 00 

Handbook of Engine and Boiler Trials, and the Use of the Indicator and 

the Prony Brake.8vo, 5 00 

Stationary Steam-engines.8vo, 2 50 

Steam-boiler Explosions in Theory and in Practice.i2mo, 1 50 

Manual of Steam-boilers, their Designs, Construction, and Operation.8vo, 5 00 

Weisbach’s Heat, Steam, and Steam-engines. (Du Bois.).8vo, 5 00 

Whitham’s Steam-engine Design.8vo, 5 00 

Wilson’s Treatise on Steam-boilers. (Flather.).i6mo, 2 50 

Wood’s Thermodynamics, Heat Motors, and Refrigerating Machines.. .8vo, 4 00 

MECHANICS AND MACHINERY. 

Barr’s Kinematics of Machinery.8vo, 2 50 

* Bovey’s Strength of Materials and Theory of Structures .8vo, 7 50 

Chase’s The Art of Pattern-making.i2mo, 2 50 

Church’s Mechanics of Engineering.8vo, 6 00 

Notes and Examples in Mechanics.8vo, 2 00 

Compton’s First Lessons in Metal-working.i2mo, 1 50 

Compton and De Groodt’s The Speed Lathe. i2mo, 1 50 

14 















































Cromwell’s Treatise on Toothed Gearing.i2mo, i 50 

Treatise on Belts and Pulleys.i2mo, 1 50 

Dana’s Text-book of Elementary Mechanics for Colleges and Schools, .nmo, 1 50 

Dingey’s Machinery Pattern Making.i2mo, 2 00 

Dredge’s Record of the Transportation Exhibits Building of the World’s 


Du Bois’s Elementary Principles of Mechanics: 

Vol. I. Kinematics.. 3 g 0 

Vol. II. Statics.8vo, 4 00 

Mechanics of Engineering. Vol. I.Small 4to, 750 

Vol. II.Small 4to, 10 00 

Durley’s Kinematics of Machines.8vo, 4 00 

Fitzgerald’s Boston Machinist.i6mo, 1 00 

Flather’s Dynamometers, and the Measurement of Power.nmo, 3 00 

Rope Driving. nmo, 200 

Goss’s Locomotive Sparks.8vo, 2 00 

* Greene’s Structural Mechanics.8vo, 2 50 

Hall’s Car Lubrication.nmo, 1 00 

Holly’s Art of Saw Filing.i8mo, 75 

James’s Kinematics of a Point and the Rational Mechanics of a Particle. 

Small 8vo, 2 00 

* Johnson’s (W. W.) Theoretical Mechanics.nmo, 3 00 

Johnson’s (L. J.) Statics by Graphic and Algebraic Methods.8vo, 2 00 

Jones’s Machine Design: 

Part I. Kinematics of Machinery.8vo, 1 50 

Part II. Form, Strength, and Proportions of Parts.8vo, 3 00 

Kerr’s Power and Power Transmission.8vo, 2 00 

Lanza’s Applied Mechanics.8vo, 7 50 

Leonard’s Machine Shop, Tools, and Methods.8vo, 4 00 

* Lorenz’s Modern Refrigerating Machinery. (Pope, Haven, and Dean.).8vo, 4 00 

MacCord’s Kinematics; or, Practical Mechanism.8vo, 5 00 

Velocity Diagrams.8vo, 1 50 

Maurer’s Technical Mechanics.8vo, 4 00 

Merriman’s Mechanics of Materials.8vo, 5 00 

* Elements of Mechanics.nmo, 1 00 

* Michie’s Elements of Analytical Mechanics.8vo, 4 00 

Reagan’s Locomotives: Simple, Compound, and Electric.nmo, 2 50 

Reid’s Course in Mechanical Drawing.8vo, 2 00 

Text-book of Mechanical Drawing and Elementary Machine Design . 8vo, 3 00 

Richards’s Compressed Air.nmo, 1 50 

Robinson’s Principles of Mechanism.8vo, 3 00 

Ryan, Norris, and Hoxie’s Electrical Machinery. Vol. 1 .8vo, 2 50 

Schwamb and Merrill’s Elements of Mechanism.8vo, 3 co 

Sinclair’s Locomotive-engine Running and Management.nmo, 1 00 

Smith’s (O.) Press-working of Metals.8vo, 3 00 

Smith’s (A. W.) Materials of Machines.nmo, x 00 

Smith (A. W.) and Marx’s Machine Design.8vo, 3 00 

Spangler, Green*, and Marshall’s Elements of Steam-engineering.8vo, 3 00 

Thurston’s Treatise on Friction and Lost Work in Machinery and Mill 

Work.8vo, 3 00 

Animal as a Machine and Prime Motor, and the Laws of Energetics. 

i2mo, 1 00 

Warren’s Elements of Machine Construction and Drawing.8vo, 7 50 

Weisbach’s Kinematics and Power of Transmission. (Herrmann—Klein.).8vo, 5 00 

Machinery of Transmission and Governors. (Herrmann—Klein.).8vo, 5 00 

Wood’s Elements of Analytical Mechanics.8vo, 3 00 

Principles of Elementary Mechanics.i2mo, 1 25 

Turbines.8vo, 2 50 

The World’s Columbian Exposition of 1893.4to, 1 00 

15 
















































METALLURGY. 


Egleston’s Metallurgy of Silver, Gold, and Mercury: 

Vol. I. Silver.8vo, 7 50 

Vol. II. Gold and Mercury.8vo, 7 50 

** Iles’s Lead-smelting. (Postage o cents additional.).nmo, 2 50 

Keep’s Cast Iron.8vo, 2 50 

Kunhardt’s Practice of Ore Dressing in Europe.8vo, 1 50 

Le Chatelier’s High-temperature Measurements. (Boudouard—Burgess.)i2mo. 3 00 

Metcalf’s Steel. A Manual for Steel-users.nmo, 2 00 

Minet’s Production of Aluminum and its Industrial Use. (Waldo.)... .nmo, 2 50 

Robine and Lenglen’s Cyanide Industry. (Le Clerc.).8vo, 

Smith’s Materials of Machines.nmo, 1 00 

Thurston’s Materials of Engineering. In Three Parts.8vo, 8 00 

Part II. Iron and Steel.8vo, 3 50 

Part III. A Treatise on Brasses, Bronzes, and Other Alloys and their 

Constituents.8vo, 2 50 

Ulke’s Modern Electrolytic Copper Refining.8vo, 3 00 


MINERALOGY. 


Barringer’s Description of Minerals of Commercial Value. Oblong, morocco, 2 50 

Boyd’s Resources of Southwest Virginia.8vo, 3 00 

Map of Southwest Virignia.Pocket-book form. 2 00 

Brush’s Manual of Determinative Mineralogy. (Penfield.).8vo, 4 00 

Chester’s Catalogue of Minerals.8vo, paper, 1 00 

Cloth, 1 25 

Dictionary of the Names of Minerals.8vo, 3 50 

Dana’s System of Mineralogy.Large 8vo, half leather, 12 50 

First Appendix to Dana’s New “ System of Mineralogy.”.Large 8vo, 1 00 

Text-book of Mineralogy.8vo, 4 00 

Minerals and How to Study Them.nmo, 1 50 

Catalogue of American Localities of Minerals.Large 8vo, 1 00 

Manual of Mineralogy and Petrography.nmo, 2 00 

Douglas’s Untechnical Addresses on Technical Subjects.nmo, 1 00 

Eakle’s Mineral Tables.8vo, 1 25 

Egleston’s Catalogue of Minerals and Synonyms.8vo, 2 50 

Hussak’s The Determination of Rock-forming Minerals. (Smith.).Small 8vo, 2 00 

Merrill’s Non-metallic Minerals: Their Occurrence and Uses.8vo, 400 

* Penfield’s Notes on Determinative Mineralogy and Record of Mineral Tests. 

8vo, paper, 50 

Rosenbusch’s Microscopical Physiography of the Rock-making Minerals. 

(Iddings.).8vo, 5 00 

* Tillman’s Text-book of Important Minerals and Rocks.8vo, 2 00 


MINING. 


Beard’s Ventilation of Mines.i2mo„ 2 50 

Boyd’s Resources of Southwest Virginia.8vo, 3 00 

Map of Southwest Virginia.Pocket-book form 2 00 

Douglas’s Untechnical Addresses on Technical Subjects.i2mo. 1 00 

* Drinker’s Tunneling, Explosive Compounds, and Rock Drills. .4to, hf. mor., 25 00 

Eissler’s Modern High Explosives.8vo 4 00 


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Fowler’s Sewage Works Analyses.i2mo, 2 00 

Goodyear’s Coal-mines of the Western Coast of the United States.i2tno, 2 50 

Ihlseng’s Manual of Mining.8vo, 5 00 

** Iles’s Lead-smelting. (Postage qc. additional.).i2mo, 2 50 

Kunhardt’s Practice of Ore Dressing in Europe.8vo, 1 50 

O’Driscoll’s Notes on the 'treatment of Gold Ores.8vo, 2 00 

Robine and Lenglen’s Cyanide Industry. (Le Clerc.).8vo, 

* Walke’s Lectures on Explosives.8vo, 4 00 

Wilson’s Cyanide Processes.i2mo, 1 50 

Chlorination Process.i2mo, 1 50 

Hydraulic and Placer Mining..nmo, 2 00 

Treatise on Practical and Theoretical Mine Ventilation.umo, 1 25 

\ 

SANITARY SCIENCE. 

• 

Bashore's Sanitation of a Country House.nmo, 1 00 

Folwell’s Sewerage. (Designing, Construction, and Maintenance.)..8vo, 3 00 

Water-supply Engineering.8vo, 4 00 

Fuertes’s Water and Public Health.i2mo, 1 50 

Water-filtration Works.nmo, 2 50 

Gerhard’s Guide to Sanitary House-inspection.i6mo, 1 00 

Goodrich’s Economic Disposal of Town’s Refuse.Demy 8vo, 3 50 

Hazen’s Filtration of Public Water-supplies.8vo, 3 00 

Leach’s The Inspection and Analysis of Food with Special Reference to State 

Control.8vo, 7 50 

Mason’s Water-supply. (Considered principally from a Sanitary Standpoint) 8vo, 4 00 

Examination of Water. (Chemical and Bacteriological.).i2mo, 1 25 

Ogden’s Sewer Design.nmo, 2 00 

Prescott and Winslow’s Elements of Water Bacteriology, with Special Refer¬ 
ence to Sanitary Water Analysis.nmo, 1 25 

* Price’s Handbook on Sanitation.nmo, j 50 

Richards’s Cost of Food. A Study in bietaries.i2mo, 1 00 

Cost of Living as Modified by Sanitary Science.nmo, 1 00 

Richards and Woodman’-s Air. Water, and Food from a Sanitary Stand¬ 
point.8vo, 2 00 

* Richards and Williams’s The Dietary Computer.8vo, 1 50 

Rideal’s Sewage and Bacterial Purification of Sewage.. .8vo, 3 50 

Turne.aure and Russell’s Public Water-supplies.8vo, 5 00 

Von Behring’s Suppression of Tuberculosis. (Bolduan.).nmo, 1 00 

Whipple’s Microscopy of Drinking-water.8vo, 3 50 

Winton’s Microscopy of Vegetable Foods.8vo, 7 50 

Woodhull’s Notes on Military Hygiene.i6mo, 1 50 

MISCELLANEOUS. 

De Fursac’s Manual of Psychiatry. (Rosanoff and Collins.)-Large 12mo, 2 50 

Emmons’s Geological Guide-book of the Rocky Mountain Excursion of the 

International Congress of Geologists.Large 8vo, 1 50 

Ferrel’s Popular Treatise on the Winds.8vo. 4 00 

Haines’s American Railway Management. i 2 mo, 2 50 

Mott’s Fallacy of the Present Theory of Sound.i6mo, 1 00 

Ricketts’s History of Rensselaer Polytechnic Institute, 1824-1894. .Small 8vo, 3 00 

Rostoski’s.Serum Diagnosis. (Bolduan.).i2mo, 100 

Rotherham’s Emphasized New Testament.Large 8vo, 2 00 

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Steel’s Treatise on the Diseases of the Dog.8vo. 3 5° 

The World’s Columbian Exposition of 1893.4to, 1 00 

Von Behring’s Suppression of Tuberculosis. (Bolduan.).i2mo, 1 00 

Winslow’s Elements of Applied Microscopy.nmo, 1 50 


Worcester and Atkinson. Small Hospitals, Establishment and Maintenance; 

Suggestions for Hospital Architecture: Plans for Small Hospital. i2mo, 1 25 


HEBREW AND CHALDEE TEXT-BOOKS. 


Green’s Elementary Hebrew Grammar.i2mo, 1 25 

Hebrew Chrestomathy.8vo, 2 00 

Gesenius’s Hebrew and Chaldee Lexicon to the Old Testament Scriptures. 

(Tregelles.).Small 4to, half morocco, 5 00 

Letteris’s Hebrew Bible.8vo, 2 25 


18 























































